The first is obviously finding out from others either by seeing them use it or them telling you.
The second is if it is used by the mob themselves as you fight.
When you hold, wear, look at some items you might receive an echo or see something in the description. You can try wearing something and fighting to see if it does something.
Also, sometimes you'll find lore about items doing something in a book or other thing in the game.
The final way is conjurer have a spell that allows them to see if an item dies something.
Mostly it just comes from experience learning one thing at a time in game or sometimes from reading logs.
Edited 1 time(s). Last edit at 08/09/2018 11:20PM by KoeKhaos.