How to measure & check the good transformer ?

How to measure & check the good transformer and the faulty one ?

kindly if anyone might upload the video for me .......... in this regard

do we have to take it out of the board ? (isolate from other components) .

what to check ........ & what should be the values ?

when checking , supply should keep on or not ?

any website that explain these things and upload the videos in these kind of matters ........... I want to learn the repairing and maintenance of electronic & electrical devices & instruments of daily use ?

thanks