Since all the code we've written so far is not dependent on the user interface, we can easily write unit tests against our classes. This step is generally taken after the first implementation of a ViewModel class. Proponents of Test Driven Development (TDD) would recommend writing tests first and implementing things afterward, so choose which method is best for you. In either case, it is a good idea to write tests against your shared code before you start using them from the View layer, so you catch bugs before they hold up your development on the UI.
Xamarin projects take advantage of an open source testing framework called NUnit. It was originally derived from a Java testing framework called JUnit, and is the de-facto standard for unit testing C# applications. Xamarin Studio provides several project templates for writing tests with NUnit.