ViewModelExample.cs

using Microsoft.Practices.Composite.Presentation.Events;
using Microsoft.Practices.Composite.Regions;
using Microsoft.Practices.Unity;

namespace SomeNamespace
{
    public class SomeViewModel
    {
    	private readonly IUnityContainer _container;
    	private readonly IEventAggregator _eventAggregator;
    	private readonly IRegionManager _regionManager;

		public ShellViewModel(IUnityContainer container, IEventAggregator eventAggregator, IRegionManager regionManager)
    	{
    		_container = container;
			_eventAggregator = eventAggregator;
			_regionManager = regionManager;

			_eventAggregator.Subscribe<SomeEventPayload>(SomeAction, ThreadOption.UIThread, true);
    	}
    }

    [TestFixture]  
    public class ShellViewModelTests {
      [Test]
      public void when_vm_is_created_then_subscribes_to_SomeEvent() {
        var ea = new Mock<IEventAggregator>();
        var vm = new ShellViewModel(null, ea, null);
        ea.Verify.... //verify that Subscribe was called on EA which Subscribe calls
      }
    }
}