Generally PVP gear has a lot more stamina than PVE gear so you can tell from their health who has more pvp gear :)

pve health usually sits about 120k-130k (unless they're a tank) and full ruthless s10 gives you about 140k-145k

I'll admit it was confusing the first time but I'm already used to it :)

Sure, but HP Pools aren't all the same for different roles. A Blood DK's HP is a lot different than a Holy Pali's. And really, I'd rather not have to research how much HP each class has per role per gearing.
Just get rid of the Mog in arena, would make it a lot easier.

While I appriciate your point, I don't see how it's much different than checking peoples buffs to see what classes are stealthed, you have to know what class has all those buffs and besides, you should already have your target before you are even close enough to tell what gear they have on and during target switches (which are generally every 30-60 seconds) you can quickly tell who is squishier.

Why should pve players get to choose how they look but we don't?

It's called a bluff. Deal with it. Card games have them. Card games are PVP as well. All games should give the player the ability to bluff or show their cards.