After i was wiped in a pug and we found it impossible to find out who was pulling first (everyone was standing near the pull point). I looked around for a mod that would tell me who entered combat first, but i couldnt find one, so here is what im suggesting. A mod designed to tell you who enters combat first - who aggro'd gruul, who accidentally aggrod supremus... that sort of thing. So they stop getting away with it. -.-
Not possible. When engaging a boss, everyone enters combat at the same time.
It might be possible to check who has aggro at the moment the boss is engaged, but this would rely on someone in your raid targetting the boss at the time, which may not always be the case if you're buffing and some mage accidentally hits his Blink key instead of his Arcane Brilliace key and pops over to have an up-close-and-personal chat with Teron Gorefiend.
Well, technically it *is* possible... but it would require you to already know the boss's exact starting position, and it would require everyone in your raid to be running an addon that broadcast their exact position upon entering combat. Since you already said you're running with a pickup group, it's highly unlikely you'd get everyone to install such an addon. :P