I need to trobleshoot the pilot generator for a Laars pool heater model lg125n

First, check to ensure that the pilot generator is fully bathed in the pilot flame. If it is not, that must be corrected first. Once the generator is fully bathed, it should put out at least 500 millivolts. New generators will easily exceed this level, but you want to see at least 500 mV. I have personally seen heaters continue to work properly with mV outputs as low as 400 mV, but spec calls for at least 500. If you are not getting 500 mV minimum, replace the pilot generator.
Testing the gas valve itself involves jumpering the valve such that the 500 mV from the pilot generator goes straight to the valve coil, bypassing all the components such as thermostat, high limits, pressure switch, etc. I generally recommend that homeowners avoid playing with the valve if they are not trained to work on gas appliances. It is one thing for a homeowner to do the simple repairs, like changing a pilot generator (which is a piece of cake for someone with even moderate skills). It is a completely different matter to play around with the valve if you don't really know what you are doing.
As a final point, unless the heater has some age to it, most millivolt gas heaters will have more issues in the components than in the valve or pilot generator. Even the pilot orifice is more likely to be a problem. I'm not saying that you don't have an issue with the valve or generator. I'm just saying they are less likely to be the problem and are always the last items I check on these heaters.