VPN encrypts data packets to prevent your ISP or the websites you visit from knowing what you’re doing online. But a VPN server’s location still leaks to the public internet, and that can be used to determine if you’re using one.

Websites can also see your detect VPNs  if they detect a pattern of behavior that is inconsistent with your regular browsing habits. They can also see it if you connect to their servers from locations that are associated with VPN services, or if they see an IP address that has been marked as a VPN user.

Peering Behind the Veil: Techniques for Detecting VPN Usage

Some governments use technology like DPI (Deep Packet Inspection) to scrutinize your traffic and detect the presence of a VPN. This technology is able to identify VPNs by looking at the encryption patterns in the data packets. The system compares those to the signature of encrypted data packets from known VPN vendors and their servers.

Another way to detect VPNs is by tracking your money trail. Many VPNs are paid for with a credit or debit card, and the PII (Personal Identifiable Information) on these cards can be used to track your activities and detect the presence of a VPN. Then there are other methods that are able to bypass the security of VPNs altogether. Some of these include detecting the location of your VPN server by comparing it to the geographic location of your residential address, or tracking the IP address that is being replaced by the VPN to identify its origin.