Create a JavaScript function that will run on the client side to check for VPN presence. While it's challenging to detect a VPN directly, you can look for signs that are often associated with VPN usage.  

Once you've conducted the checks, you can update the application's interface accordingly. If the checks indicate that a VPN is detected, you can display a message or change the interface elements to notify the user. If no VPN is detected, you can proceed with the normal operation of your application.