Are you able to detect if someone is preventing port scans?

The point of port scanning is to detect if the customers run some kind of web server to enable their IP as a proxy. To connect to an IP to use it as a proxy (and spoof IP/geolocation), some ports need to be open. If there is no port open because they spoof it to avoid detection, we can still detect if the IP is a server by looking at the type of the ISP.