Consider this class:
public class Content
{
public virtual bool IsCheckedOut {get; private set;}
public virtual void CheckOut()
{
IsChec
Can I suggest that you might be thinking about this in the wrong way - generally you should be setting something up, performing an action and then checking the behaviour (result). In this case does it really matter that it wasn't set to false by the setter - what should matter is that it is false in after a given scenario has been exercised. If you take tests in isolation this might seem a bit odd, but for anything your tests will exist in sets.
The situation would be different if you were testing the interaction between two classes - then it would be fine to set up an expectation on the property setter - as the setting action is the interaction you're testing.
I'm not familiar with Moq as I use Rhino.Mocks - but I'm guessing there'll be something along the lines of mock.VerifySet(content => content.IsCheckedOut=It.IsEqual(true));