A compression check will tell you a lot.

You're not going to like this, but huge amounts of well pressurized blowby rarely comes from engine compression. Usually its from a spun engine bearing. The bearing spins, vaporizing the crankcase oil, producing huge amounts of crankcase pressure.

A big key in this is the appearance of the blowby. If it's hazy appearing, it's almost assuredly from oil vaporization. If it's crystal clear, then combustion gasses are more likely.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)