How do trees know that animals even existed in order to eat their fruit?

Here is a plausible chain of events (based on basic reasoning and zero knowledge of actual plant biology)Step 1: plants evolve seeds.  I'm going to take this as and arbitrary starting point, just to focus on your question.  When we're talking about evolution the only true starting point is a lifeless planet, but I hope we can all agree that it isn't worth rehashing the entire history of existence every time we talk about evolution.Seeds carry nutrients, the way that eggs do for animals that lay eggs.  The seedling needs energy to grow and doesn't have any way of producing it yet, so it needs to come with a bundle of food to eat.Step 2: Animals evolve a tendency to eat the seeds because hey, free energy.Step 3: Plants evolve tougher seeds because if fewer of your seeds are eaten, then more can survive to make adult plants.Step 4: A plant evolves strong enough seeds that some portion of them pass undigested through an animal.  If that potion is only, say 5%, the animal will continue to gain benefit from eating the seeds, but the ones that aren't digested will be planted in some high-quality fertilizer in a location the plant normally couldn't reach.  This is huge!  This plant goes everywhere, spreads faster than any other species, is the first to colonize new areas, and just generally wins.Step 5: Some descendant of this plant evolves a little tasty covering that attracts more animals and improves its propagation by animal.  This is also hugely successful, but energy-intensive so mostly it only works for plants that can construct a massive energy harvesting engine, which is mostly trees, though bananas are a pretty amazing exception.As you can see, each step is simple, yields a short-term benefit, requires zero 'knowledge' on the part of the life form in question, and lays the groundwork for the next evolutionary step.  This is how evolution works.

Trees don't "know" these things. The trees with the random mutations which produced fruits had their fruits eaten by local animals and their seeds distributed more widely than trees which did not have these random mutations. Therefore, the trees with the fruit mutations (fruitations?) were naturally selected for.

The first fruits probably had nothing to do with animals.  When the fruit falls, it rots and provides fertilizer for the new plant.  Plants that had fruits with more sugars attracted animals that spread the seeds.  They were more successful and outcompeted the plants that didn't have such succulent fruit.  Feces also makes good fertilizer.

The trees respond to the existance of animals because it works, Evolution doesn't work by knowledge, it works by blind stumbling around until something works, and whatever works gets passed on.  In the case of plants, they develop seeds in their ovaries, which fall and grow into new plants. Animals that eat plants will sometimes eat the seeds, which means the plants get spread farther around and and propagate more. The plants who's ovaries are the most appealing to animals (in terms of being larger, more tender, sweeter, more colorful, etc) will have their seeds eaten more commonly and reproduce more effectively, eventually drowning out all those plants that don't. Eventually, the ovaries become fully external, and grow larger and more nourishing, because those that do get spread and those that don't, stagnate. The tree never decided to do any of that, they just changed a little at a time, by random, dumb, blind mutation, and those mutations that worked ended up spreading, until they developed to the point where they were almost unrecognizeable.

Unlike the others, I will interpret your use of the word "know" metaphorically (or very generally), to mean that the tree species, and the natural selection process that is operating on the tree species, is somehow getting information about the environment.  The tree isn't thinking about it, of course, but natural selection does need some sort of feedback, which comes in the form of a change in reproduction rate. This whole process would have happened very gradually.  The plants whose seeds had coverings with nutrients that could be used by animals, tended to reproduce slightly more then those that didn't, because animals spread the seeds.  The plant didn't have to produce a giant tasty fruit to get results....it just had to have a little bit of a seed covering that was nutritious.  This change could have easily developed by a chance mutation. So, the slight statistical increase in reproduction rates for trees with nutritious seed coverings vs. those without, was "known" to the natural selection process operating on that tree species.  Keep in mind, there was no "first fruit," there was just a long slow transition to seeds with no coverings, to seeds with thick, sugary coverings that we now call "fruit."  At all steps along the way, reproduction rates varied, and that information affected the natural selection process.

If we are anthropomorphizing, let's clarify.... Trees don't "know" a thing about animals, as they lack anything resembling a brain. The DNA of a tree does, indeed act in a manner that is intelligent, but only of you think of the world from the point of view of DNA. That's a really hard mental trip to take. But the fact is at any given time, DNA is in an optimal state for reproduction based on all prior history of the system it emerged from. That is, for all practical purposes, intelligence. Nature engineers brilliant solutions through trial and error in a competitive environment. It is a massive supercomputer playing a strategy game. So again, does this DNA intelligence actually "think" something like "hey let's try putting our seeds in a tasty fruit so those animals will eat them"? No. But the construction of such a sentence is heavily biased towards what we consider intelligence -- the manipulation of language and symbols. Those are very high order emergent effects of complicated biological interactions. The same way it seems unlikely that a lump of neurons could think up this question, it also seems unlikely that evolution could come up with clever solutions. But they are the same, just on scales of time and space so far apart it is hard for us to conceptualize.

Nature is a zillion times "worse" than Monsanto!  Nature has genomes, radiation, chemicals, comets meteors volcanoes, climate, viruses; all sorts of possibilities that can randomly modify any random organism. which it callously does as fast and as irrationally as it possibly can. Every so often,  some of the little monsters are similar enough to be fertile together. and maybe maybe not they meet and maybe reproduce and maybe produce surviving offspring. Kinda bigger odds than winning a billion dollar Powerball maybe. Not so anyone notices. Sometimes something succeeds for an eon or for an age, and rarely, even longer. Eventually one or another of natures seemingly random playthings, like an astroid, scraps almost everything above the size of a shrew or maybe a hydra, (successful or not) and starts the process all over. Everything gets recycled though... nothing is lost. No one no thing gets made out of brand new material so far. We are all, every nano-bit of us, roughly 14.6 billion years old next,Tuesday (or maybe Saturday) So nothing to worry about. We have a free ride on a self-maintaining autonomous universe... until something better comes along. That about as cool as it gets! Does the tree "know" We are too young a species to be ready to answer that question yet... neither the data nor the maturity and wisdom exist yet. But I wouldn't bet against it other than if you specify that trees must know things just the same way as humans do for the hypothesis to be validates.. And that seems to be an unreasonable demand.. Can't say yes, can't say no.

Your question subtext suggests you think trees are, or once were, conscious and self-aware - not so! Evolution 'invented' fruit as a means of dispersing seeds, and didn't have to 'think' about it in terms of being 'aware' that animals existed - it's own action on animals ensured that, when some new kind of food appears, something or other will eat it!

The trees doesn't know these things, all  they care of is to manufacture their own food so they can survive.   That is why they are on the Kingdom Plantae because they are different in the Kingdom Animalia.