In which layer should the network availability check be done in a xamarin based mobile application code?

I'm having a Visual Studio solution with the type of projects,

• PCL (Portable Class Library)
• IOs
• Android

I follow the MVVM pattern as well.

I want to know where I should add the network availability check in this code base? Is there a design pattern for this? All services (or network based calls) reside in the PCL.

It would be really helpful if anyone can provide an architectural based answer with some design patterns for me to check with