I`m going to buy used HAP ac RB962UiGS-5HacT2Hnt.
Any case to quickly test that device was not damaged?
maybe some self-test could be run?
p.s. i has no previous experience with Mikrotik, but work with another network equipment.

I think by resetting it to defaults you should be able to test all interfaces (wired and wireless).
Perhaps do a RouterOS and firmware update to check if memory is working as well..