Do a compression test in conjunction with other assessments, such as evidence of blow-by, oil consumption, power on the road, etc.
For example, on my trip south winter of '22 the engine began using oil which got worse as the trip progressed. Downwind of the road-draft tube was a mess of mixed oil and dust. Power wasn't down much but some. By the time I got home it was using a quart in 300 miles. I did a compression test and compared it to previous tests. No. 6 cylinder was down to 110 pounds from its previous 135, while the highest was 140. Previously, no. 5 was the lowest at 120 from a scratch in the cylinder wall that occurred somewhere in the past. The plugs showed no signs of oil burning. Given 200,000 + miles on the shortblock I decided to pull it apart. No. 6 piston was burned back about 1/2 inch from the edge and about an inch long yet it still showed 110lbs but there was a lot of blowby blowing oil with it out the vents.
Oil in the cylinders from leaking oil rings can mask worn rings by still showing normal cylinder pressures on the compression test. Likewise leaking valve-stem seals.
