I don’t know why this got downvoted, this is actually a really solid answer for cheap piston compressors. Ring blowby and a lot of pressure behind the oil cap or air / oil shooting out the breather is a sure sign of ring failure. You check used car engines like this too.

You can also check the valves to make sure they are sealing properly.

Ex car mechanic, current industrial mechanic here. I work with big boy compressors.