Have Raypak versa B-055B MN heater. Pilot lights but burners will not fire. What to check first? How to determine if valve needs replaced?

Check the water pressure sensor in your heater.

You need to start by hooking to one terminal and folowing the wire to each componenent on the line the problem will be with the one that fires the heater. You will need to look further once you find the component. For example if you find the pressure switch to be the problem it may be a dirty basket or a rusted water header or just a bad switch. So each component will only be a symptom. Good luck.

what burner where on the oven or the cooktop, if its the oven it may need new gas valve w\ thermocoupler

check mv output of thermopile ,unless you have 350-500mv it is not enoughvoltage to power valve