In order of actuation, my Suburban furnace goes: time delay relay switch, then sail switch, then limit switch, then gas valve and ignition board at the same time. Therefore, you can check the sail switch by checking for 12 volts DC at the live side of the limit switch, and check the limit switch by checking for 12v
at its ground side, when the thermostat is calling for heat. On my furnace I can check these without pulling the furnace. The limit switch is at the front left, clearly visible as a silver disc attached by two self-tapping screws. I have to be careful not to short out the probes to ground while doing this. Insulating tape round all but the very tip of the probes helps.The limit switch can be checked for operation with the power off by testing its resistance. The limit switch can be manually operated by pressing the circular part in the centre. This sometimes jams in the open position. Its function is to go open circuit if it detects excess heat. If you have 12v
on the ground side of the limit switch, you should hear the gas solenoid click sharply about 15 seconds after the fan first runs. If no click, then the gas valve may be defective.When I had the same problem, it turned out to be a defective ignition board. I replaced it with a Dinosaur Universal Ignition Board, which has fuse protection, 3 tries for ignition, and is soldered on both sides for coping with vibration and change of temperature. Good luck. Nick.