There is a video on youtube called how do we know the universe is flat. (put those keyword in youtube and you should find it, I can't post links to videos until I have 10 posts) It has something to do with using the Earth as an apex of a triangle then measuring two points on the cosmic microwave background. I'm assuming they then add the angles of the triangle up to 180 degrees. Well, how do you know what the angles of your triangle are? I've seen three tries at attempting to explain this. No one succeeds. There is also something about going around the Earth in a balloon in Antartica and I don't see why that is necessary.