Telling a guy's circumcision status has been an interest of mine since my teenage years.
This is more of an art than a science, although there are blatantly obvious cases where the rim of head make a distinct imprint.
It helps to know the guy's pedigree ( age, where he was born, ethnic origin, whether parents were born in USA or europe). There is also some body language hints. Just hints. As I said, it is an art rather than science. This is what is called "foreskindar".
In underwear, there are some uncuts whose foreksin is thin and glans large and generate a glans imprint leading to erroneous classification of "cut"
Generally, the shape of the tip of glans in underwear gives good hints.
There are some circumcised males whose skin bunches up at rim and there is little or no imprint of shape of rim, in such a case, you need to look at the shape of the tip of head for any hints of a foreskin. Even in speedos, some guys can be ambiguous.