$10-15 will get you an outlet tester at just about every home improvement store. You plug it in and the three big LEDs light up and you compare it with the sticker on the device. Get one with a GFCI tester built in, when you press the button it will short to ground and if your receptacle has GFCI protection or is on a GFCI protected circuit should trip the GFCI protection.
Most power line adaptors say to keep it on the same circuit. The one I have is running a small VoIP phone and I don’t have issues with call quality.