High pressure sodium fixtures have both a capacitor and an igniter. The igniter operates a "tickler" circuit that makes the ballest produce voltage spikes up around 2KV to 5KV until the lamp ignites. If you have a good ear you can hear the noise the igniter produces in the ballest when you have a bad lamp or open circuit. The igniter is typically a small circuit board with plug-on terminals, a plastic frame that plugs in - will have 3 or 4 small components, or a small (half dollar size) tubular part with 3 wires(usually) The igniter is visably smaller than the capacitor.
The first test is try a new HPS lamp. If this fails try a mercury or metal halide of the same wattage, or a 200-300 watt incandescent with a mogal adapter. If the lamp comes on the igniter is bad. Don't do this any longer than necessary to prove a good ballest.
After a HPS lamp has been used a long time it winds up producing an internal gas pressure that requires more voltage than the ballest can supply to maintain the arc. This causes "cycling" where the lamp comes on, heats up, then goes back out. The older the lamp is and the longer it has been in this mode the faster this will happen. If you are doing maintenance on a group of lights turn them on for 15 or 20 minutes to see if any are beginning to cycle.
DO NOT put a meter across the socket connections if the fixture won't light. If the lamp is bad you will see the 2-5KV spikes across your meter.
It's been a while, but if I recall there is a point around 150 watts where the voltage changes from around 88 volts (up to 150 watt) to 100 volts (150 watt up) nominal on the lamp, and the 150 watt is made in both voltages - double check the info on the ballest AND the lamp in these.
Again, it's been a while but it seems to me that the max the ballest will supply to the lamp is around 108 volts, and you can predict lamp failure by checking the voltage supplied to it. Since it's been a while, you can verify the voltages I gave by installing a new lamp in a fixture and checking the voltage after an hour or so, then install a lamp that cycles and check the voltage continuously till the lamp goes off. Anything up in the volt or two of the cut off voltage is a candidate for replacement.
Finally, a yellow iodine color on the inside of the lamp is evidence of a leaking gas tube.