Only certain boss mobs and named monsters somewhat consistantly drop the same loot. Thottbot.com already has all of that information on hand including % chance to get the loot you are after based on the all the times the mob has been logged via Cosmos. Using the Cosmos UI keeps their database updated with historical info on EVERYTHING any specific mob has ever dropped. I seriously don't understand why a site would want to duplicate the work another site has already done particularly well (thottbot) and yet another site is already trying to copy (stratics).
If it isn't blue, and it isn't from a quest, it's random. (Well, some things seem to drop in select places, like Pearl Encrusted Spear, but mostly its random.)