How do I avoid driving into my garage with my bike in the roof rack?

Get rid of the automatic door opener in your car. Having to get out of your car to open the door will force you to notice if the bike's on top.

Can you put up a mirror on the back wall of the garage. As you drive in, you would see your car and what's on top of it.

One of the problems with an ordinary photoeye is the standoff distance; where would you mount it so that you could get an alert beacon and be sure that you would see it before advancing into the garage ? Another problem is the photoeye range; diffuse sensors can transmit a few feet. To cover a 20 foot driveway, you would need a through-beam style or a reflector (recently linked to an inexpensive one, above), and that means another tower or location to hold the reflector or transmitter. An industrial laser zone scanner would work ! But they start around $3000. I've never seen a general-purpose infrared proximity sensor that could distinguish between a car with a bike on top and a car without a bike on top. If you usually store the bike in the garage, I would try to enforce the habit of moving something large into the middle of your parking spot every time you remove the bike. Seeing a sawhorse (or another bike, or a traffic cone) in the middle of the parking spot will remind you the bike is on the roof.

I was going to say basically what Sweetie said. When you put the bike on the roof, remove the garage opener from the car or put it in the backseat or something, then when you're like "Why is my opener in the backseat???" you'll be like, "Oh yeah, bike on the roof."

Get an aftermarket backup sensor meant for parallel parking and attach it to the front of your bike whenever you mount it on the rack. It will beep in the car when the bike is about to hit something whether that something is your garage, a drive-through overhang, or a parking structure.

Weight sensor would never work - how would it possibly recognise empty car+bike but not go off for car with 3 friends and no bike, or even car with groceries?

Put up a large sign that hangs down inside the door. IS THE BIKE ON THE ROOF?

Replace roof-mounted bike rack with trailer-hitch mounted bike rack.

I dunno, I think people suggesting the "cone" or other reminding techniques are maybe not the kind of people who'd drive into a garage with a bike on top of their car. I, however, am. I have left my laptop at home, like, 30% of the times I've brought it home. It's more than a 1-hour round trip to get it. I try a lot of techniques to remind me. I figure if I have 3 or 4 tactics then probably 1 of them will work. Some of these help a bit, I still forget it. That is, if you were the kind of person to remember to put out a cone every time you left with the bike, you'd not be the kind of person to ram your bike into the garage when you left it up top without remembering. I do actually think the most practical solution is "don't use a bike rack that puts your bike in a position where it'll get ripped off the top of your car" but that isn't what was asked.