But if the VPN provider really was routing other customers traffic through your internet… you’d know it. You’d see requests and traffic that you didn’t make going across your router. Since you’d be the endpoint for the other dudes VPN you could probably even see what IPs they are connecting to and get an idea about the nature of their requests.

I dunno if SSL encrypts the entire HTTP payload or not but could you even figure out the URL’s being requested using a tool like wireshark?