Differs by age. Penis should look "normal circumcised" when performed on an adult. Babies and little boys often look as if they have only been partly circumcised due to prominent pubic fat pad which pushes penile skin forward so it can look uncircumcised. Temporary adhesions can develop which seperate with time. Push down on pubic skin on both sides of penis to see if there is xs skin. "bridging" is abnormal.
Appearance. Appearance is probably the best way to tell. If there is still a significant remnant of the foreskin, then the circumcision was probably incomplete. If the sutures were not placed correctly, there could be strictures causing too much tension on the head of the penis. Don't be too quick to judge however if the circumcision is very recent; the skin will need time to heal properly.