I've seen a video (probably on youtube, feel free to look for it) in which a guy basically straps a camera to a balloon and then lets it go.
It flies up and up and up until you can actually see the curvature of the earth, that seems like a good test you could undertake if you want to disprove a lot of astronauts :)
Also check out codyslab:
I thought this was a pretty cool and easy to accomplish test
you can't see the curve in the video. he spends 15 minutes explaining to you why he thinks what he thinks is a curve. he didn't send up a balloon either.
here are high altitude pictures
can't find one with a curve that is not also a fish-eye lens