Best way to identify your opponents' class out the gate?

So far I've been fairly casual at PVP, and a lot of the reason is because I have trouble coordinating my kills, especially with pug teams. One of the biggest reasons for this is because I often can't tell the difference between blm/whm/smn. This is especially true if they are lalafells, shapeshifted, don't have their weapon shown, or aren't wearing a standard robe.

What should I be looking for. If I run out and see three casters out the gate, how do I know 100% that my target is the SMN, without wasting any time? I need something quick and easy to look for for each class, that helps me identify them without wasting a moment.

there no good answer here, it really depends on a lot of factors.

You can tell whm/sch from the dps casters cause their i70pvp (looks like a weaver O.o kinda) and i55pvp (grey overcoat looking thing) gear is different looking than the DPS caster i70 (vanya). If they are wearing pvei90 its easy to tell.

If you cant tell by clothing, the weapon is next best- Purple Book = SCH Green Book= SMN WHM staff is obvious, and BLM staff is also obvious. It gets tricky when they have allegan , books there is no way to tell apart really, allegan staff is always whm cause BLM allegan is a 1h usually coupled with the primals shield.

You can also tell if its a blm or smn by whether they have aetherflow stacks, or the astral fire/ice stuff up(later in fight) many people also watch closely to see which pet is following whom, you can tell sometimes that an eos or garuda will follow their masters direction shortly after they move, so watching for pet movement will pick out specific casters at times.

All lalafell teams are the hardest, especially if they r all allegan, or all pvp similarly colored, at times there is no way to tell, just start hitting one of them till they pull their weapon out and be ready to switch on the fly

GL out there, that my best try at an explanation