network – Is there a device I can plug into an ethernet port to see if it is connected and live?

I want a device with a standard ethernet cord that I can plug into jacks to see if they are alive and providing network. I don’t want to have to run to the router to see if the light is on (for a loop-back device), and I don’t know where the other end of the cord goes (so I can’t use the 2-part cable tester that sends a signal from one side of the cable and detects it on the other side). I want to be able to plug in to an ethernet outlet and immediately see if there’s live network or not (I don’t need to capture packets per se). Is there any such thing, and if there is, what is it called?