What Dave said is how I judge compression. When the engine is turned over if a definite resistance is felt when the piston is nearing top dead center then there's usually enough compression to start the engine. If it feels like what I call 'soft' when it gets to TDC then a person should start checking for the problem. I would say 90% of the time it's leaky valves. Over the past many years on this forum it seems the primary reason for 'soft' compression is from valve leakage as opposed to piston blow-by.
At one time I had made a miniature compression tester but with the very small displacements I don't know how accurate it was. There are some commercially made compression testers that won't even work on small engines, the lawnmower, weedeater type. The reason for this is because they don't have the check valve, that holds the compression, right at the tip of the adapter so when the engine is checked the amount of area in the hose has to be added to the volume of the cylinder thus reducing the compression pressure.
gbritnell