I agree with tail wheel, pilot that can accurately diagnose a problem for the engineers is highly respected by them.
Rather than stating, left engine running rough. State which mag. Then when you drop it off, tell them when it started, how bad, and what stage of flight if applicable and then how you diagnosed the problem. They should always go and run it themselves but they go straight to the problem. Saving you and them time. Could also save you money as well.
Simple stuff, but a couple of correct calls in a row and they will begin to trust your judgement.
Oh, and if either one makes a call thats it's not airworthy, then don't accept it.