would you be able to tell it's a Socket A, as opposed to my Socket 939?
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.
Just google up images man