One way to check for blow-by is by doing a compression test on the engine. Cylinder pressure should be within 10% of factory specs. on an engine with no problems. You could check for excessive blow-by by hooking a pressure gauge up to the dipstick tube. Any reading of more than 1 or 2 psi. would indicate bad rings.

A compression test is not a good test for blow-by. The cylinder walls can be badly tapered and still show good compression but the engine will have alot of blow-by. The proper way to check blow-by is to use a leakdown tester. The gauge initial setting (calibration) is with the piston of the cylinder being tested at top dead center and the valves closed. Then rotate the piston to bottom dead center and the valves closed and test again. There is no such thing as perfectly sealed or "No leakdown". While the test is being performed with the piston at bottom dead center. Listen at the Carburator/throttle body, dipstick tube, the exhaust pipe and the radiator cill location. The sound of air at the intake/exhaust manifold can indicate a valve or head problem. At the dipstick you will hear air which will be what is going past the rings (blow-by). Seeing air bubbles in the radiator could indicate a blown head gasket or a cracked block. The use of a leakdown tester does require the use of an air supply such as from an air compressor. Any professional race engine builder will tell you that a leak down tester will tell you more than a compression test.