(nothing else useful to add)Honestly if you're buying one and the seller doesn't know. I would just go ahead and replace those items. They aren't necessarily hard to do, just time consumingA lot of the typical 7.3 issues are tough to find without a meter and testing continuity, resistance etc. CPS, Under Valve Cover Harnesses, glow plugs, glow plug relay etc.
Check for oil around the turbo pedestal. Could indicate bad seals on either the turbo, the pedestal or EBPV.
If 4wd see if the auto locking hubs work. The vacuum lines tend to crap out and leak. The plastic hub dials get old and brittle. I plugged mine and put Warn Hubs in.
I see this as a foolishly optimistic test. Every engine has crankcase pressure, that's why they put a vent on them. And try balancing an oil cap on any running engine and it will probably vibrate off too. I've had people do this on great condition trucks I'm selling that don't use a drop of oil over 10k miles, and say that it was worn out and had too much blowby. Of course since its on the internet, people are gonna believe it.check and see if you have air pumping up out of the oil filler cap. Take the oil cap off and turn it over and sit it over the hole. If it gets blown up/off, the enginis on its way out...
. Make sure oil pan isnt a rust pit.