Unique Weapons

Is there anyway to determine whether an enemy will have a specific weapon before fighting him/her?

Yes, to an extent. The weapon that the graphic is weilding is the weapon that they can drop, and will have the mods that that weapon has. So, if it's Vampiric, you'll have extra -5's popping out of your head. Or, if it's a specific Green you're after, and you know what it looks like, you can look at the boss to see if they're weilding that specific weapon.

Thank you for the quick response!