For some sources of damage it is pretty easy to guess what damage type it is. For example, if a monster hits you with a huge ass axe, it's probably physical damage. The burning floors at the butcher is most likely fire damage. The chilling construct monsters in act 2 seem like they inflict cold damage, etc.
However, the damage type is not so obvious for all attacks. Belial's ultimate skill (the green colour seems to indicate poison, but the sudden explosions might indicate fire, and Asheara yells "Watch out for the fireballs" in the quest prior to Belial, where similar green explosions occur). Azmodan's black pools on the floor. Zoltan Kulle's whirlwinds.
How do I find out what damage type a certain opponent is able to inflict, so that I know what type of resistance is most important?