Json

_controller.WithCallTo(c => c.Index()).ShouldReturnJson();

_controller.WithCallTo(c => c.Index()).ShouldReturnJson(data =>
{
    /* Assertions on the data being turned into json (data), e.g. if you are using NUnit, you might have something like: */
    Assert.That(data.SomeProperty, Is.EqualTo("SomeValue");
});