public class MyClass {
    private readonly static MyClass _instance = new MyClass();
    public MyClass Instance { get { return this._instance; }}

    private MyClass()
    {
        DoSomething();
    }
}

[TestMethod]
public void TestMethod1()
{
    var fakeSingleton = Isolate.Fake.AllInstances<MyClass>();

    Isolate.WhenCalled(() => fakeSingleton.someFunction()).WillReturn(true);

    Assert.IsTrue(MyClass.Instance.someFunction());
 }