One thing you can do for this is actually to make your debuff bar really big and to focus your co-tank (and make the focus bar big, as well). When you see a debuff on them, hover over it with your mouse and quickly read the tooltip. If it's something that would make them take more damage that has a timer on it, that's generally an indication that it's a tank swap mechanic.
Of course the absolute best thing you can do is ask your co-tank if they know if the fight you're doing has a tank swap, and at what point you should swap (and discuss whether or not you want to use Shirk - it can be done, but it doesn't feel all that essential). As others have said, the best way to swap is to use Tank Stance (if you're a little early with switching from DPS stance to tank stance, it won't hurt you that badly) > Enmity Combo 1 > Provoke > Enmity Combo 2 > Enmity Combo 3.
EDIT: Also wanted to say that there are definitely edits coming to the first few posts! There have been some excellent suggestions in here for things to include, and I've already got outlines drafted. I'm so glad this is helping people, and I'm also really glad to have other folks contributing to the discussion.![]()