I know for sure my stuff is toast when it no longer sizzles when I hold it up to my ear and prying the post up has no effect. That means that one or both of the wires on the coil has broken off. Up to you if you wanna try to rebuild it (recoil/rewick).
Dry-burning does help in bringing them back to life, but you have to clean and dry them first. If it has filler in it (cartos, nanos), you run the risk of burning the filler. Once you do that, the burnt taste is forever, and you really won't be able to tell 'til you try to use it again (wasted liquid, but <1ml, so no big deal). I have dry-burned cartos before, but they have to be clean and completely dry, and you have to do really short bursts, stopping as soon as you see a glowy orange dot through the tiny hole on top. Then you have to clean off the ash by cleaning them all over again. Dunno if they're worth all the trouble to you. Also with dry-burning, if you do it for too long or too often, eventually the super-thin coil wire breaks, then you're left with the problem above.
I'm broke and have to get a lot of mileage out of my stuff, so I actually do go through all this trouble. Don't worry about burning wicks. They can take a lot of heat. It's popping the wire and burning the filler or any silicone that you have to worry about. Really short bursts, and stop as soon as you see the orange glow. Unfortunately, it's the only way I know of to get rid of the nasty carbon crud that builds up on coils. No amount of soaking in any kind of liquid (denture tabs, alcohol, etc.) has done any good for me. T3s are probably the easiest to revive, but I'm a carto girl, so...too bad for me.