Like the Dog and others have said, use your charts or cruising guides to look them up, if that fails, do a drive by. You might find a staff member. If its a small marina and not congested or busy, you should be able to communicate with them with out even stopping. If you don't find a staff member, see if they have a temporary dock to tie up to. Just don't approach a strange marina unprepared at night.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White