How can you judge when a non-stick pan is the correct temperature for pan frying?
You shouldn't heat a non stick pan without the oil, because it is too easy to overheat it and ruin the coating. 

So, I put oil inside first, and tell by the state of the oil. It smells, looks and flows different when it is hot enough. You can also learn to judge if there is sufficient heat when you hold your palm at a given distance above the pan, but in my experience, you learn to judge by the oil before you have gotten good enough to keep the same distance and to distinguish "enough" from "not enough" heat. 

If you don't have enough experience to know when the oil is ready, you have to do it repeatedly until you learn it. It takes a few less-than-great meals if you do it purely by trial and error, and goes better if you use an infrared thermometer. You can also try floating cubes of stale white bread and watch their browning behavior, but I find this method too involved logistically - you have to have the bread, cut it, then deal with messy pieces of oily bread, and end up with slowly charring crumbles in your pan.