How can I test ModelState?

You don't have to use a Mock if you're using the Repository Pattern for your data, of course.

Some examples: http://www.singingeels.com/Articles/Test_Driven_Development_with_ASPNET_MVC.aspx

// Test for required "FirstName".
   controller.ViewData.ModelState.Clear();

   newCustomer = new Customer
   {
       FirstName = "",
       LastName = "Smith",
       Zip = "34275",    
   };

   controller.Create(newCustomer);

   // Make sure that our validation found the error!
   Assert.IsTrue(controller.ViewData.ModelState.Count == 1, 
                 "FirstName must be required.");

This not only let's you check that the error exists but also checks that it has the exact same error message as expected. For example both of these parameters are Required so their error message shows as "Required".

Model markup:

//[Required]
//public string Name { get; set; }
//[Required]
//public string Description { get; set; }

Unit test code:

ProductModelEdit model = new ProductModelEdit() ;
//Init ModelState
var modelBinder = new ModelBindingContext()
{
    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(
                      () => model, model.GetType()),
    ValueProvider=new NameValueCollectionValueProvider(
                        new NameValueCollection(), CultureInfo.InvariantCulture)
};
var binder=new DefaultModelBinder().BindModel(
                 new ControllerContext(),modelBinder );
ProductController.ModelState.Clear();
ProductController.ModelState.Merge(modelBinder.ModelState);

ViewResult result = (ViewResult)ProductController.CreateProduct(null,model);
Assert.IsTrue(!result.ViewData.ModelState.IsValid);
//Make sure Name has correct errors
Assert.IsTrue(result.ViewData.ModelState["Name"].Errors.Count > 0);
Assert.AreEqual(result.ViewData.ModelState["Name"].Errors[0].ErrorMessage, "Required");
//Make sure Description has correct errors
Assert.IsTrue(result.ViewData.ModelState["Description"].Errors.Count > 0);
Assert.AreEqual(result.ViewData.ModelState["Description"].Errors[0].ErrorMessage, "Required");