How do I know when coconut oil is ready to go?

Probably the most dependable is to drop a small piece of bread or some such in and check the sizzle. I usually settle for getting my fingers damp and flicking a few drops into the pan- it should sizzle and evaporate almost immediately. There are other methods- you can tell a lot by viscosity (how easily it flows if you tilt the pan), color etc.- if it starts smoking you've gone too far for most things. Any way you do it, some experience is very helpful, but basically it needs to be hot enough that your food starts cooking immediately.