How can I tell what type of damage an enemy is dealing? [duplicate]

Talking about those Swarms, I know they are Physical damage.

However, for most others, you can usually tell by the color of the spell that is being used:

Green - poison Blue - ice/frost Red - fire Purple - arcane

Although this is not always 100% true, you can usually guess.

Armor would help all of these stats, but then again it'd take a lot to raise all of these resistances drastically.

What I'm trying to say in a nut shell is: Are you farming, or making progression? If your farming, boost the resistances you need (i.e. Fire / Physical if you were farming Diablo). Typically I try to balance all of my resistances so I can progress.