It looks like the illustration silverwings posted. Basically, if the man has to roll back a piece of skin to reveal the head of the penis, he has not been circumcised. In my experience almost all American men are circumsized. The only uncircumsized men I have met were eastern European
Generally if a guy is uncircumcised, his foreskin will be removed completely, The appearence will not have any skin, while an uncircumcised one will have a foreskin. There is a major misconception that if you are uncircumcised, the foreskin will always cover over the head of the penis. In a lot of cases this is true but they are all different and don't all look the same. some have foreskin that extend even a little past the bell end (sorry didn't know the scintific term lol) while others might be just about over, others half way and others even underneath it.
You can tell easily if its limp. If you can see the head, the pinkish "helmet" without anything over it, he's circumcised. If it's got a hood of skin over it, he's not. Once it gets hard it harder to tell.
If it looks like a huge earthworm, he isn't circumcised.