One day we were out trying to kill some pirate boss in the lower decks of his ship but there were players who would kite this boss once and somehow hide and let other players (in this case us) finish him off. (I think they put a ranged shot on him and while the boss made his way around the walls to the player, he would first encounter us who thought one of our party had hit him and would gleefully kill him off and then wonder why no one could loot him.)
Durn taught me something that day that really helped. He pointed out that once a mob has been attacked by anyone who is NOT IN YOUR PARTY, his bar changes from Red or Yellow to grey. Sure enough, that boss's bar was grey and we couldn't get the credit we needed.
It was hard to picture what he was talking about at first so here are some pic's to explain.
Red or Yellow Bar = No one has attacked this Enemy yet:
Note - this holds in a PVE environment and you'll notice that PVP targets may show differently.