How can I know which mobs have loot?
is it random?
All mobs have loot correspoding to their level , a lvl 2 mob have a chance to loot a tier 2 armor and yes ,it is random, and has far as i know all mobs of a same tier can loot an any amor of the tier
Complementing myrani reply, the loot chance varies depending on monster tiers and the odds of getting higher quality items gets better depending on how high is your level compared to the monster you are fighting.
My source is this video which has the exact % and more information that may help you