Jonathan de Halleux shows the simple way to test a singleton. Given this sample code:

using System;

public sealed class Singleton
{
   ...
   private Singleton() {}
   public static Singleton Instance
   {
      get { ...}
   }
}

You can test it with reflection like this:

using System.Reflection; 

[TestFixture]
public class SingletonTest
{
    private Singleton target = null;

    [SetUp]
    public void SetUp()
    {
        ConstructorInfo ci =
            typeof(Singleton).GetConstructor(
                BindingFlags.Instance |
                BindingFlags.NonPublic,
                null,
                Type.EmptyTypes,
                null
                );
         Assert.IsNotNull(ci);
         this.target = (Singleton)ci.Invoke(null);
    }
}