It's not a stupid question at all. I read that in surveys wives often are wrong when reporting their husband's circumcision status. Indeed an intact friend of mine once had his partner ask him if he was circumcised or not. When erect the foreskin usually rolls back and it is quite hard to tell. If skin covers the bell-shaped head (the glans) when soft then the man is almost certainly intact. Also if when erect you can stroke the entire length of the penis using the gliding action of the skin then he is probably intact. Circumcised guys also have a scar about an inch below the glans. If there is no scar, the skin forms ridges behind the corona of the glans when retracted and is attached under the pee hole under the glans, then he is almost certainly intact.
Hope this helps,