I never mentioned compression (which would be checked with the plug installed). I was talking of checking the "pinch" at TDC. The pinch at TDC is an easy indicator of the engine's compression- this is very easy to check without the added expense of a compression gauge. Usually if you have some "pinch" left at TDC, you have adequate compression (life left) in your engine.

If you want to check the compression of your engine, you can buy a $30 gauge from dynamite or try this:

remove the engine from the car and get the piston on it's upward stroke towards TDC. support the engine by holding only the flywheel and see how fast the engine rotates around the flywheel. If it falls right through TDC while holding the flywheel the seal (compression) is gone. if it slowly rotates through TDC your still OK. Obviously a new engine will not roatate past TDC with htis method.

sometimes it helps to put a little MMO in the exhaust port and rotate the engine. watch for bubbles to form as the piston travels up. you'll get some bubbles as the piston crosses the exhaust port, but if you see bubbling all the way to TDC you are losing compression in the sleeve fit.

As said above- also if the rod bearing is gone you will not be getting the full stroke of the crank, and hence compression will be very low (like adding shims).