If a circuit fails, it's simple to test a single-pole switch and see whether the switch is at fault. Testing a circuit with a three- or four-way switch, however, is more complicated. You must determine which switch, if any, is bad. No matter what kind of switch you test, you use a continuity tester and a bit of logic. You have to remove the switches to test them, so make sure you turn the power off first. Mark the wires as you remove them so you know how to rewire the switch.
About 10 minutes for a single-pole switch; up to 1 hour to test three four-way switches
Screwdriver, continuity tester
Reading a continuity tester
Turn off the power, take out the switches, and mark the wires so you know how to reconnect the switches.
Replacement switch if necessary
Test a three-way switch by clipping the tester to the common terminal, which is a different color from the two traveler terminals. Attach the probe to one of the travelers. If the switch is good, the light goes on and off when you flip the switch. Attach the probe to the other traveler -- if the switch is good, the light turns on when the switch is in the position opposite the one that turned it on during the first test. It should go off and on as you flip the switch.
A four-way switch has four terminals and two toggle positions. One toggle position connects one pair of terminals; the other position connects the other pair. Usually the pairs of terminals are at the top and bottom of the switch housing rather than on the same side. You can find the pairs by trial and error for testing if the switch is not marked. Start by clipping the tester to one terminal and attaching the probe to any of the others. If the tester fails to light, flip the switch. If it still doesn't light, move the probe to each of the remaining screws until you find a pair of screws that turn on the light.
Repeat on the other pair of terminals. If the switch is good, flipping the toggle turns on the test light when it is connected to these terminals. The continuity tester should light for only one pair of terminals at a time.