if it is running you can run cpu-z to get all of the info. if not you could look for model name imprinted on board and search online for it. I am sure if you have looked at a ton of boards you could tell the difference. There are so many different types out there it is tough for me to tell just by looking.
Yeah, it's pretty blatantly obvious between a socket A and socket 939 dude. For one, if you're able to actually see the CPU the socket 939's are smaller and have the heatspreader. Socket A's dont have a heatspreader, only the single core in the middle and the actual CPU wafer itself is bigger.
Of course if theres a massive heatsink on there, you might not be able to really see the CPU at all, however the locking mechanisms for the CPU would be different as well between each socket.