As others have alread said a leakdown test and an
oil analysis by someone like Blackstone labs.
The leak test will tell you the condition of the cylinders and rings and will find problems well in advance of a compression test. The
oil analysis will tell you what if any parts of the engine are eating themselves or being eaten.
When you do the leakdown test you want to use one of the old Sun Leakdown testers or one of the new Snap-On testers. They are the only testers sensitive enough at low leakage percentages to show you what you actually have for cylinder/ring leakage.
Parts store and internet units that are cobbled together from air pressure regulators and various pressure gauges seem to be the same but lack the low leakage calibration and measuring precision of the Sun and Snap-On units that is necessary to accurately measure cylinder leakage.
Ed