Select the other tank as defensive target so you can watch his stacks, usually you switch at 10 stacks or so.
A VERY usefull addon is Effect UI, and you should grab it for many other purposes as a tank (keeping track of your CDs ? watching for casts you need to interrupt ?) and for any other role too. It lets you see big and clear in the middle of your screen whatever buff/debuff/cast you want to see. You can customize size and color and set special allarms …