As I understand it, in principle, a ramp check should establish that the rules are being complied with. The rules will be a combination of ICAO and any national or regional rules. If there is a rule that says avionics tests should be done then it is fair game for a ramp check. If that means getting an engineer to demonstrate compliance with the rules, so be it. Well, that's the theory. The practise is, no doubt, rather different in some cases.
It's not my area of expertise but I have had some peripheral involvement with the SAFA programme. I understood from those doing the ramp checks that they would routinely limit their enquiries to paperwork unless that elicited evidence of non-compliance.