If you have the manifest card in the back of the warranty book, the left column should have an entry that is entitle mtr. nr.
Additionally, the diesels and maybe most all engines have the motor number on the drivers side rear of engine, below the head, and above the oilpan rails.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in