You can remove the flame sensor and test it if you want but it sounds like it is not very old.
The way i test it is to clip an ohm meter (multi meter set to ohms)to the two tabs on the flame sensor.
Hold the window of the flame sensor against a 60 or 100 watt incandescent light bulb.
After about 30 seconds the switch inside the flame sensor will open. (open circuit)
Pull the flame sensor away from the light bulb and the switch will close again after several seconds.
You will be able to hear and feel the switch when it opens and closes.
A heat gun or led light bulb will not work you have to use incandescent bulb.
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