From what I've read there are a few possibilities on where the blowby could be coming from on my 6.2. What is the probablitiy of highest to lowest?

Rings
head gasket
cracked piston
cracked head

I don't know but I have blowby on my '94 and it recently cracked a piston. I could tell something was wrong by all the oil dumping out of the dipstick tube. I got 0 psi compression when checked on the bad cylinder. I swapped it out and have less blowby now but it's still there. These high compression engines blowby where and when they can. I've put near 4K miles since the piston swap.
A compression test will show you if you have anything to worry about and which cylinders to look at once you pull the head(s). should be ~400 psi across all 8.

I've got a compression gague for a gas engine. I haven't looked at it in a while and couldn't tell you how high it registers. would that work for a diesel? are the injector ports or glow plug ports a standard spark plug thread?

I did a search. Answered the gague question myself. most gas ones only go up to around 200psi. No where close to diesel ranges. Anyone know a cheap place to get one?

A little more info. The engine is to run well. Just a lot of blowby from the oil filler. Would that point more to rings/headgasket? A cracked head/valve would point to poor engine performance wouldn't it? Sorry for all the questions. I'm new to diesels.