Address Collision Questions

How would we know if there is an address collision?

More importantly, if someone generated an address that another person already had and it had BTC in it, could that new person theoretically remove the bitcoin from the wallet without the other owner knowing about it?

I understand that the number of possible addresses are so big that is unfathomable that it will happen in the foreseeable future, but these are questions that would be good to get answers to for the uneducated.