1 - TTL detection: If the value of TTL is less than the expected value, they can assume that there is tethering. However you can tweak Windows / Linux to send a higher value than usual.
2 - User-agent sniffing and OS fingerprinting: When your browser requests a page, it sends its user agent, version and other parameters. Networks can detect if the version of Chrome or Firefox is a desktop version. The other thing that is easy to detect is OS. Check this if you want to know how much they can detect of you.
3- Traffic/port sniffing: Networks look at what UDP / TCP ports you are using and what kind of traffic is going over them. So they can detect videogames, bitttorent, etc.
Using a VPN you can at leat disguise your traffic in a way that it is harder to apply all these techniques.