Tool to plug in to an ethernet jack to sense whether it is active
I don't know what to call this, and have thus far had no luck googling an answer. Ideally, I'd have a small meter that would plug in to a female RJ45 wall jack. It would give me a green light (or whatever) to indicate that there was an active network available.
There must be a tool that does this? What's it called and where can I find one?