Oten time you can unscrew a panel in the freezer to look at the coils. If they're covered in frost and ice, the defrost system is not working. Also there is often a fan to move cold air from the freezer to the frig. That fan can also often be observed by removing a panel.
I used to have an old frig with a manual defrost system. Button inside the door. Push it to go thru a defrost cycle. Liked it.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13