Can I check the odometer reading on a '94 C280 in the junkyard?
 

Without the key of course. If I put 12V to the battery cables will pressing the trip odometer reset button show the mileage without the key?

Wouldn't a 1994 C280 have a mechanical odometer that can be read anytime without the need for a key or battery? My 1995 E320 works that way. Hard to imagine the C-Class cars having a more sophisticated instrument panel before the E-Class cars.

'95 E-class is the last of that bodystyle made since 1986. The '94 C-class is all new with an electronic digital odometer.

Our 1998 E320 with an electronic digital odometer shows the mileage when you open the driver's door ... no need for the key. So, assuming all the associated wiring is still in place, hooking up a battery should allow you to read the mileage in the 1994 C280 junker.