Innokin's really good about battery safety. Unless your individual device is defective (not impossible), it should have an internal temperature sensor that prevents it from operating when it gets too hot. It'll be difficult to tell with a built-in battery, but the general rule of thumb is that if you can't comfortably hold a battery in a closed fist for 30 seconds, then it's too hot. Also, hands can be deceiving. A mod that's been in a pants pocket, for example, warmed up to body temperature, can feel pretty darn warm if your hands are icy cold. Try touching it to your lips or cheek, instead. If it's still warm enough to make you go "whoah," then maybe you put it down for a few minutes and use something else while it cools off a bit. Otherwise, you should be good to go.