Every proxy checker I've come across answers one question: did the connection open?

That's the least interesting thing about a proxy. A proxy can connect perfectly and still hand your real IP address to the destination server in a header. If you're scraping, testing geo-restricted behaviour, or just trying not to be identified, "it connected" tells you almost nothing.

Here's how to actually check, with code you can run.

Three ways a proxy gives you away

1. It forwards your real IP in a header