You guys know the hammer vs feather drop on the moon?! I think that proofs that we've been there! Just watch it on youtube if you don't know what i mean. Search for hammer feather moon or something.

I.e.g the principal is like this. Gravity lets stuff accelerate at a certain speed per second (on earth 9.81 m/s). As such, things are bound to fall at a simultaneous speed, even with a different weight or mass. However, the Earth has an atmosphere. Within that atmosphere is air (combination of oxigen, hydrogen, nitrogen etc...) This creates friction with the thing that's falling. So really light things fall less hard because the can't move the molecules in the air as fast aside as the heavy falling obstacles can. That's why there's a difference in speed. On the Moon however, there is no atmosphere, so no air, so no friction... As such, Newton thought things of a different weight would fall at a simulatious speed. He was right according to the youtube video. And i think this is proof enough that we've been on the Moon!