You may also see a rise in the resistance. When my 1.8Ω coils go up to 2.2 or higher I know it is about time to change it. I use protank/evod type coils. I keep the used ones in a small bottle of pg. when I get several in there I take them apart, clean them and then rebuild them. Even if you aren't into rebuilding right now you may want to keep your used coils for future rebuilding.