Incandescent Christmas lights are popular because they burn brighter than LED lights, and the bulbs can be individually replaced if they burn out. As a general rule of thumb, any bulb larger than your little fingertip is going to be an incandescent bulb. Smaller bulbs, however, might also be the newer LED lights. Here is a surefire way to tell which is which.
Plug in your string of Christmas lights. You should be sure to plug them into an inside outlet for this test, since you do not want external factors like outdoor temperature affecting the outcome.
Allow the lights to warm up for about 10 minutes. You can simply leave them coiled on a step stool or chair while they warm up. If you want, you can take this time to note if any bulbs are out or missing.
Put your thumb directly on top of a bulb. If it is almost hot to the touch, it will be an incandescent bulb. If it is almost cool or barely warm to the touch, it will be an LED light. Once you know what type of lights you have, you can proceed with your Christmas decorating and be sure that all your replacement bulbs are the right type. The small, incandescent lights will never give off enough heat to burn skin, so while you should be cautious, there is no reason to fear getting burned.