Compression is easy to check on the ground. Also you could warm up the engine and sample the emissions. The real test would be to measure power output using a dynamometer, although difficult unless the installation intended the required instrumentation.
The easiest way for existing installations would be to measure power required to hover. Of course, you'd be subject to the accuracy and precision of many data sources (pressure altitude, temperature, manifold pressure, aircraft weight, wind, ...). Unfortunately, this would also detect changes in drivetrain losses, so not really what you're asking for, but still quite useful for the pilot.
Is this a question for which you already have a good answer?