If you're using a glass banger, the best way to tell if it's hot enough is to look at the color of the dab. If it's clear or light yellow, it's probably not hot enough. You want it to be a dark amber color. Another way to tell is by touch. Put your finger on the side of the banger closest to the heat source and count to three. If it feels too hot to keep your finger on for more than three seconds, it's probably ready. If you're using a quartz banger, you can hold a lighter up to the bottom of the banger and wait for about 10-15 seconds before applying your dab.