Plug the "string of lights" into the power point. If a particular light does not come on, then it is "bad".
The load on a string of Christmas lights is typically located in the bulb itself. Each bulb is designed to produce light and consumes power when electricity passes through it. The collective load of all the bulbs on the string determines the amount of power required to illuminate the lights.
Christmas tree lights, this parallel circuit prevents one bulb failure from turning off the whole string of lights.
Animated Christmas lights can be made several different ways; the most popular is to use a flasher bulb at the end of the string so that the lights blink in a certain sequence. These can be purchased in sets as well as coming with specially marked sets of lights.
it depends on what kind of Christmas lights you have
One common reason why half of a string of Christmas lights may not work is a broken or loose bulb causing a break in the circuit. Another possible cause could be a damaged wire or a blown fuse in the string of lights. Sometimes issues can also arise from a faulty socket or plug connection.
With series Christmas lights, if one burns out, the whole string stops working. With parallel Christmas lights one light burning out only affects that light. This makes it much easier to replace burned out lights in the case of the parallel lights.
Santa
With the modern lights nothing happens.
Remember the old Christmas lights that were a simple circuit. When you plugged the string in, hopefully, they would light up. If they didn't light up, you had to take a proven good bulb and screw it in to each light socket on the string until you found the faulty bulb. With a string of lights wired into a parallel circuit, you or wouldn't have to do that. You could identify the faulty bulb right away because it would be the only one which did not light up.
You could have a bad bulb or loose bulb. Check out the reference page I am giving you.
series