Gauge your engine!! You'll want; Oil Temp, Coolant Temp, Engine Oil Pressure, and transmission temp & pressure (if possible). All of this along with the Tachometer. Then add in the if you can the low level oil and coolant
alarms along with the other temp
alarms.
With the gauges you can log your engine's operation temp & Oil pressures, while in use. Then any changes will stand out like a sore thumb.
An Hour meter also for those pesky oil changes at regular intervals.