I know it's a camera angle, but how do I MEASURE it?

Aaahhhh - I get it. I understand the importance of getting some basics right, but isn't this really a question about what Photoshop (or your PP software of choice) can do with the two images? I think this would be quicker, easier, and more reliable than trying to take two compatible photos, the power of this sofware is such that the manipulation possibilities are truly amazing (to me anyway). I would never try what you are attempting, I would go the PP route (assuming that the image of the person is roughly ok). After all, what you are proposing is exactly what the perjorative 'photoshopped' means lol. My (final) suggestion is to ask this question on a photo forum (DPreview's retouching forum for example), where people with more experience/skill on the software than I have would be happy to give you suggestions on what is possible/how to do it. Even if you proceed with your method, this would give you a fallback for finetuning the merge. Hope that helps, and good luck with it!

use a tripod?

Yes, use a tripod. I've also seen camera sticks that just allow you to steady the camera on the ground, and conveniently at the same height. If you can't, make sure you hold the camera at eye level, and then match the horizon to some physical feature on the camera next to the viewfinder.

"Perfectly" level? Big ask. Apparently you do not have a camera that does this for you, but if it is important to you, that would be the most effective way of solving the problem. Otherwise you are going to have to carry a level around with you, or at least with your camera. The other solution is to learn to use the levels provided for you by nature and the construction industry in the features and characteristics of the scenes you are photographing. The ability of digitals to show previous images is a big help here. On review, I am not sure what you are trying to do here. This is normally an issue for panoramas, where multiple images are joined to create a wide angle image, or for landscapes where the horizon (obviously) should be level. However, you speak of 'another picture of someone ... taken at the same angle' - are you overlaying one image on the other? Have you looked to see if what you want can be done in post-processing?

I wonder if there's an app for that. Seriously, the iPhone certainly has gadgetry for judging orientation wrt gravity. Of course, you either have to use the phone's camera or strap the phone to the camera somehow.

GeeEmm: looking for a way to go "ok, so this was pointed at 45 degrees upwards, so when I take the other picture I need to have it at a 45 degree angle so they look similar enough when I composite together. except I don't know the angle so I need a way to figure out the angle. Squeak: yes, that, indeed!

Mephron, I simply look at what is in the top left hand corner (or bottom if that is better) of the viewfinder, and then move the camera so that is then in the top right corner (or bottom as appropriate), and repeat as many times as necessary... Inclinometer would be more precise, tripod (with levelling bubble) and swivel head best though. Good luck.

Let me try to make it simpler: 1) take photo of building 2) take photo of person 3) composite together in photoshop so it looks like the person is leaning against the building If you can't get a reasonably similar angle of shooting, it's going to look ridiculously fake and the illusion is going to fall apart. That's what I'm trying to do.

If your camera has a flash shoe, then you can use a http://www.bhphotovideo.com/c/product/263729-REG/Manfrotto_337_337_2_Axis_Flash_Hot.html that mounts there.