That's a very practical question, my dear friend. To check Xmas light bulbs, you need to follow these steps: First, make sure the lights are turned off. Next, remove the bulbs from the sockets carefully to avoid breaking them. Once you have the bulbs in your hand, inspect them for any signs of damage such as cracks, dents, or other physical damage. If there are no visible signs of damage, insert the bulb back into the socket and turn on the lights. If the bulb works, then it's good. If not, then replace it with a new one.