Actually you test from the lead ends at F anf Terminal at the protector relay.
There are 2 reasons for a trip:
One is flame is not there thus the cad cell faults the control.
Two is that flame is there but the is a mechanical fault with the cad cell or protector relay.
So the bid part is finding if we have a problem with flame being present or flame faulting.
You can do any testing of the cad cell from the protector relay instead of under the transformer.
Cad cell testing manual
Now as far as flame leaving _ this can be bad coupling slipping on pump, bad pump, vacuum leak, etc
The process is eliminating and setting a schedule of fault,
I found one that only faulted in the morning,,,,it was a vacuum leak that after the unit stayed off for 4 hrs or more the fuel migrated back into the tank cauasing an early am shut down,