How do you tell the difference between 2 bolt and 4 bolt?

assuming you mean with the engine intact-
code on side of block, code inside glovebox of original vehicle the engine came from.
original crank pully will have a marking on it meaning 4 bolt main. not sure what the marking is but its different between 2 and 4 bolt mains.

if you mean looking at the block with the heads off the caps holding down the crank have 4 bolts to hold each cap, a 2 bolt main has well...2.