One of the best bits of advice I got early in my career was to note the exact position of critical engine gauges especially the oil pressure. One day a student and I were climbing out for an aerobatics lesson in the club C150A. On this airplane the oil pressure stayed pretty constant at about 2/3 scale up the green arc, however i noticed it was in the lower half of the arc and then watching the needle it made a slight but noticeable drop. I told the student to immediately turn back to the airport. By the time we got to short final for a downwind landing the oil pressure was indicating zero so I shut down the engine and we made an uneventful power off landing.
It turned out that a tooth on the oil pump drive broke off and over the course of a very few minutes the skipping gear ground off the other teeth until the pump was no longer turning. If we had continued the climb to the aerobatic box we would have had a forced landing in relatively inhospitable terrain instead of an uneventful landing on the runway back home.
After that I would randomly cover the engine instruments with my hand and ask the student to tell where the needles were sitting. "in the green" was not a satisfactory response.
Engines almost always give you some warning of an impending mechanical failure and any sudden change in oil pressure or oil temperature almost always is the first indication your day is going to get interesting.....