Gas dryer not firing up and all the components of the burner check out good but you're not sure how to check that flame sensor on the side of the burner tube? Samurai Appliance Repair Academy Fellow Budget Appliance Repair (a.k.a., Willie) reveals the wax on-wax off moves for testing the radiant sensor in a gas dryer.
The full way to bypass and test if the radiant sensor is the problem:
1) UNPLUG DRYER FIRST!
2) remove both wires from the radiant sensor
3) bypass the sensor by sliding the two connector terminals together and make sure they are away from everything
4) plug the dryer back in and set to a timer position that should heat.
5) start dryer and and watch for igniter to start glowing.
6) if the igniter glows wait about 10 to 12 seconds and pull the two wires apart carefully making sure not to touch the ends or short them to anything.
7) if all else is OK with the dryer the flame should light up when you separate the two radiant sensor wires
8) if the above all works OK and it didn't before then replacing the radiant sensor should fix your problem.
Here's the part link for the replacement radiant sensor ==> Flame Sensor
This video shows you how to replace it: