There is a leadership AA called Inspect Buffs, that lets you see debuffs (which includes DoTs) on a mob. You need at least three people in group (and a merc counts) to make use of it. When you're not in a group, you can test the duration of yours with your gear and AAs, and learn to keep tabs on it mentally. It's a habit thing, use it enough and you'll get a feel for when it's time to recast. It's usually a good idea to have a window with resists and wearing off messages split to it so they don't get lost in battle spam- important for when things resist slow, all your buffs wearing off someone at once usually means they died, et cetera.