Hey Deigo! Unfortunately, there's no foolproof way to detect VPN usage as VPNs are designed to mask the user's actual IP address.
Mixpanel uses the IP address of the incoming request to determine the geolocation properties. If a user is connected to a VPN, the location for the geolocation properties will be based on the location of the VPN server that the user is connected to.
If you notice a significant number of users with their geolocation properties set to a specific country that doesn't align with your expected user base, it's possible these users are using VPNs or other methods to mask their actual IP address.