How to know if it is a failed transmission or failed pcm.

Hello, Sorry to hear about this. The PCM will not cause a grinding noise. It is just a little circuit board with a lot of wires attached. The transmission bearings could have failed, the engine crankshaft or rod bearings could have failed, or the issue could be related to something like a bad wheel bearing, etc.. Lots of things to make a grinding noise on a car. If the technician has inspected it to find that he/she cannot turn the crankshaft manually, this would be grounds to consider the engine or transmission for failure.