Basically you need to check that it is working.

You are looking for a switch that is activated by the flipper linkage. Manually operate the coil (with the power off) and ensure that the switch is activated just before the flipper reaches it fullest travel. You can use a multimeter with probes at either side of the switch. Put the multimeter on the continuity setting (or on low Ohms if you don't have one on your meter). I believe the Williams of that era use a normally closed switch, so the meter should read zero (or buzz) and then go open (infinity) when the flipper reaches full travel. If I am wrong it is the opposite way around, but either way you are looking for the switch to change from one to the other.