The main difference between a circumcised penis and an uncircumcised is the presence or absence of a foreskin (a sleeve of skin present around the head of the penis). During circumcision, the foreskin of a penis is removed surgically, which exposes the head (glans penis). While in an uncircumcised penis, the foreskin is intact as it was naturally present.
Another indicator of a circumcised penis is the scar on the underside of the penis. If you are circumcised, you will see a mark near the head of the penis.