Cheap and nasty check for a head gasket is to run the engine for 10 minutes , turn it off, wait a minute the remove the dip stick.
Smoke signals = blown head gasket.
Between the vylinders is also the breather.
A blocked breather will also cause the engine to smoke.
A bad oil scraper ring will make no difference to the compression or the short term running but will cause the engine to smoke although this will usually load up the plug.