Unit testing for an event using Reactive Extensions

I'm using Reactive Extensions for .NET (Rx) to expose events as IObservable<T>. I want to create an unit test where I assert that a particular event is fired. Here is a simplified version of the class I want to test:

public sealed class ClassUnderTest : IDisposable {

  Subject<Unit> subject = new Subject<Unit>();

  public IObservable<Unit> SomethingHappened {
    get { return this.subject.AsObservable(); }
  }

  public void DoSomething() {
    this.subject.OnNext(new Unit());
  }

  public void Dispose() {
    this.subject.OnCompleted();
  }

}

Obviously my real classes are more complex. My goal is to verify that performing some actions with the class under test leads to a sequence of events signaled on the IObservable. Luckily the classes I want to test implement IDisposable and calling OnCompleted on the subject when the object is disposed makes it much easier to test.

Here is how I test:

// Arrange
var classUnderTest = new ClassUnderTest();
var eventFired = false;
classUnderTest.SomethingHappened.Subscribe(_ => eventFired = true);

// Act
classUnderTest.DoSomething();

// Assert
Assert.IsTrue(eventFired);

Using a variable to determine if an event is fired isn't too bad, but in more complex scenarios I may want to verify that a particular sequence of events are fired. Is that possible without simply recording the events in variables and then doing assertions on the variables? Being able to use a fluent LINQ-like syntax to make assertions on an IObservable would hopefully make the test more readable.