In practical terms for PUGs I generally assume whoever has the higher unbuffed health tanks. Usually whomever has higher health is better geared with the exception of pentamelded accessories. And if they are using pentamelded accessories just let them stretch their epeen and DPS anyway. Barring that, if they won't go tank stance (or turn theirs off!) then I'll go ahead and do it. It makes me happy when the other tank steps up. Not because I get to DPS, but because then I get to do all the voke/shirk and external cooldown stuff most people ignore when they OT.