The extension offices do test your gauge for free. I do it a couple times a year. But thats just your gauge. I'm talking about the whole thing.
Older canners from someone you don't know personally are not recommended because of safety concerns. Much like car seats. You don't know if they've been in an accident. So they check for things like microscopic cracks, pits, dents. Things that you can't necessarily see but that can compromise the safety. It's been a few years, so I don't know if this is still offered. If I remember correctly they could also fix some things.