How does su detect fakeroot?

If I run su, it asks for the password of the target user. If i run it as root, it does not ask for a password. But if I run it in fakeroot, how is it possible that it will ask for a password? Should fakeroot not make all programs think that you are root? For my own program, the only way of doing this would be trying to run su, and look at the return code. but how does su do that?