Checking if a device is safe to use
With respect to grounding, the key property is, that all metal parts that are exposed and can possibly be touched by a person, have to be connected to protective earth. With a multimeter in beeper mode, you can check if all of those parts are connected with the main plug's protective earth contact.

But depending on the kind of device, there are other things that can potentially go wrong and be a risk. For example, I once had to do with cheap chinese usb wall chargers which used really low quality transformers with questionable galvanic isolation. If something goes wrong in this case, your life depends on the rccb working properly.

In my opinion you need some experience to really qualify whether a device is safe to use or not and you may have to look at the electronics inside.