The Dometic fridge recall is quite easy to tell if it has been done by inspection. Among other things, it adds additional sheet metal shielding around the burner area. If you need to unscrew a sheet metal panel to get at the gas control valve, the recall has been done. If you're unsure, any RV repair place authorized to do the recall should be able to tell you if it needs doing and, of course, should also be able to do it at no cost to you.
If this is a private party sale (and it sounds like it may be), then I'd generally trust the owner's word that they had a particular recall done absent any documentation one way or the other. Calling the relevant companies with the VIN or serial number, or looking the information up online if available, is another way to verify in many cases.