Decompile the apk using https://www.decompiler.com/

Detecting Xamarin Apps
Detecting Xamarin apps on Android usually involves identifying specific files commonly found in Xamarin applications. These are .dll files and .blob files, which are part of the Mono framework used by Xamarin.

https://medium.com/@justmobilesec/introduction-to-the-exploitation-of-xamarin-apps-fde4619a51bf#:~:text=Detecting%20Xamarin%20apps%20on%20Android,Mono%20framework%20used%20by%20Xamarin. Use this link for more detail and credit goes to this website.