It's not currently known exactly how Pokerstars, and other poker sites, detect VPN usage. It's likely to be one of the three following methods, however: Their website logs and stores known VPN server IPs, and when a connection is made from one of these IP addresses, the site knows that it's VPN traffic.
Pokerstars uses various methods to detect and block VPN usage by players. These methods include: Blacklisting known VPN IP addresses: Pokerstars maintains a list of IP addresses associated with VPN servers. If a player attempts to connect to the website using one of these IP addresses, they will be blocked.