How do you know if a boy if circumcised?

If his penis head shows all the time, even when he's soft, then he is probably circumcised. The presence of a circumcision scar is the surest tell tale sign of circumcision.

If his penis head is covered by skin when he is soft, then he is uncircumcised. The skin can be pulled back, and it frequently slides back off the head as a man starts to get an erection.