To check a fuse for a string of lights, you will need to remove the fuse from the socket and test it with a multimeter. To so: 1. Turn off the power to the string of lights. Then, remove the fuse from the socket. 2. Set the multimeter to the "ohms" setting and touch the leads to the fuse. 3. If the multimeter reads "0," the fuse is good. However, if the multimeter reads the same value as when held apart, the fuse is blown and needs to be replaced.