Every IP address has to be registed to an ISP, VPN servers are pretty trivial to detect as you'll see that the registration info from ARIN, RIPE, APNIC, etc. comes up as "HideMyAss" or "Amazon".

There are companies out there that provide paid anonymous, reliable (not hacked) proxy servers that run on consumer internet connections though, and those would be very hard to detect as vpn / proxy connections.

Another way is through capabilities where you have a computer that identifies itself as a macOS device but has the TCP/IP stack of a windows device – that's a proxy ! I would recommend the source code of p0f or the book silence on the wire to learn more about the topic.

I would start with public proxy lists like HideMyAss. I can't imagine foreign users pay premium for VPN services like IPVanish, etc.

What % of VPN users will that get you? 10%? This seems a very hard problem to actually solve...

Try getIPIntel.net