www.dieselgiant.com
Go to the MB maint page.
Scroll down to the last video, blow by. This shows a perfect engine.
On your car: pull the air cleaner and look for oil & deposits. Filter should be clean but may show some darkening where the breather tube enters.
If your blow by looks anything like a 2 cycle engine, DO NOT BUY! Exhaled strong puff on a cigar, DO NOT BUY. Steam coming off my head when ... ok. Smoke made by a cig in an ashtray, ok.
Try this at idle and 1k RPMs.
Strong blowby can cause a runaway diesel!