With returns, people don't always give feedback on what the reason for the return was. Either they didn't like the device or operation or potentially something was defective...
Items are tested when they are returned, but I wouldn't set my expectations too high, it will probably just be a quick check to see if everything that came with the device is still there and maybe a quick function test.
If something is missing from the scope of delivery, the buyer will probably not be refunded the entire amount, so it could be that something is also missing. Well, with an RME device that would be no big deal .. Power-, USB-, TOSLINK- cable can be purchased if needed and the manual is available in a current version for download. And you surely can get a printed manual from RME as well if you also want printed documentation.
So you could start your check by checking if the scope of delivery is complete. See manual chapter 2 "Package Contents".
https://www.rme-audio.de/download/fface_ufxplus_d.pdf
https://www.rme-audio.de/download/fface_ufxplus_e.pdf
In case a hardware defect was the reason for the return I would check the device all around for optical damages or if there are audible loose parts inside (turn it carefully in the air or shake it slightly, but carefully).
After that I would test all analog inputs and outputs to see if any of them are not working. Inputs could possibly have been overloaded once.
Also check all buttons and knobs.
Then I would test USB3 and Thunderbolt connection to the computer to be on the safe side, if you have both interfaces on the computer.
Digital interfaces could be checked with a loopback test. Configure the device as clock master (actually default), connect ADAT1 IN to ADAT1 OUT, do the same with ADAT2 and AEs and check in the driver settings if a synch occurs.
Check the headphone outputs and maybe MIDI and the rear USB port for ARC USB and the front one for DURec.
I think after such a thorough check you are done with the topic.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13