The best way to test a pilot generator is under a realistic load. Robertshaw milivolt gas valves have a 12 ohm load on the magnet, so I have a twelve ohm resistor that I connect across the pilot generator leads, then connect my meter in parallell with the resistor.
A minimum acceptable read is about 350 milivolts, while a new pilot generator and a good pilot flame generates about 500-550 milivolts.
Occasionally, you find pilot generators that give high reads with no load, and zero reads when connected across a load. This testing procedure is more accurate and reliable than testing without a load in my experience.
What type of generator do you have ? (voltage 750 250 ie pg-1 pg-9 robertshaw honeywell) The way I have always taken reads is with a closed system (calling for heat pilot on)A- read take across generator terminals C- take across t'stat terminal B- this is what is left to operate valve take across operator terminals. When taking only an open read (not calling for heat) you will never see if the generator is breaking down under load. reads taken inder load also will show if there is excessive resistance in the system such as a poor splice,wrong t'stat loose wire etc. If we don't know what reads to expect it makes it difficult to troubleshoot.
When taking millivolt reads remember to take a closed read. I only take an open read to confirm an open in the system. (an open will show the same read as an open or closed read) I have seen many open reads say in the case of a thermocouple show 22 or so and a closed read of 0-1. Which of course is dead. Same thing with a P.G system. May show 500+ open and as low as 0 closed.