You can find that the flow is coming from the Phonepe Switch by two things :
a.) UTM Source
You can embed the utmsource = phonepe in the URL and share us the same to us as a Static URL so you will know that the flow is coming from the Phonepe switch platform.
To know more about utm source please find the link here : https://www.launchdigitalmarketing.com/what-are-utm-codes/
PhonePe embeds a unique User-Agent value of “phonepe-webview” into the browser using which you can detect if the user is inside the PhonePe App Webview.
You can get “phonepe-webview” as a user agent inside Phonepe webview
You can find more about User Agent here : https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent