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?
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Best Answer
The only true way to find this out is testing. You could easily do this on normal.
You should do multiple runs with 0 Resist All to get the control information. I would say a sample size of at least 10 kills. Average the damage from each fist attack (or whichever attack of his you want to figure out) across all runs and that is your control figure.
Once you have the control, you can start introducing variables like each separate Resist stat. I would start with Physical Resist, preferably a very high amount, so you can more easily see the impact it has without the difference falling within the margin of error. Do the same amount of runs and average the damage once again. Technically, if the attack indeed is Physical, you should see a reduction very similar to the amount of reduction your Resist stat is giving you. The more runs you do and the larger the sample size is, the more accurate this will be and the closer the actual reduction you see will be to the amount the stat says it will reduce by in the character sheet.
You will most likely have to spend gold on the proper gear to do this, but it is the way people have found out many of the games mechanics and nuances. FRAPS or some other form of recording is also a good idea so you don't have to write things down during the fight.
This may not be the easy answer you wanted, but it is the scientific method, and it's how humans have been figuring things out for years! Especially theorycrafters in video games!