First a nitro engine has pinch and compression. They are related but different. Pinch is the tapered fit of the psiton in the sleeve. Compression is the air fuel mixture being compressed once the pinch seals off the combustion chamber.
You can have great pinch and zero compression but you canot have poor pinch and have good compression.
You can check you engine (w/o running it) in either of two ways.
First, using just 1 finger, turn the flywheel one full revolution. There should be a point where the flywheel gets very hard to turn. On a newish engine you should not be able (or nearly not) to turn the engine past that point with just a single finger. If it turns easily all the way around then the piston/sleeve is worn out and needs to be replaced.
The second way is to dissassemble the engine and take the piston and sleeve and try to push the piston through the sleeve from the bottom. You should not be able to. In fact, if the piston does not stop at least 1/8 of an inch from the top of the sleeve then they (the p/s) are worn out and should be repalced.
Before you start ordering parts do yourself a favor and compare new engine prices against the parts costs. Often it's better to just get a new engine rather than rebuild. With a new engine you get, well a new engine including a warranty rather than a well worn one with a couple new parts.