Location

If I understand you correctly, you would like to see if person A is sharing their location with person B using person C's device (whether or not it belongs to you). If so, unfortunately, this is never possible. Location sharing information is private for the intended recipient only.

When you share a location you can share it from the device you are currently using; if someone chooses “People” in Find My they will see where you are based on your selection. But you can always check the location of a device by going to Devices instead of People.