I was taught a good way to check compression or better yet the ability to make compression.
Remove glow plug and turn engine over with finger turning the fly wheel counter clockwise. Look at top of piston and when it is at top of stroke there should be a fairly tight feeling/fit. This will tell you how well worn the piston and sleeve are. Some engines will continue even when way worn but only one I new of was an RB WS7 ( he claimed 11+ gallons on same p/s).