ReactiveUI ViewModel Unit Testing how to test that a Invoke comes from a certain WhenAnyValue

Question

I am trying to write a unit test that tests my ReactiveUI ViewModel. In my ViewModel I want to retrieve comments from my database. You can also filter these comments by text and by creator.

For filtering I use the command this.WhenAnyValue like here:

Observable.Merge(
     this.WhenAnyValue(vm => vm.CommentCreatorId)
       .Select(s => Unit.Default)
       .Throttle(TimeSpan.FromMilliseconds(300)),
     this.WhenAnyValue(vm => vm.SearchText)
       .Select(s => Unit.Default)
       .Throttle(TimeSpan.FromMilliseconds(700))
 )
     .InvokeCommand(this, x => x.GetComments);

So I have a subscription for CommentCreatorID and SearchText that calls GetComments when it is changed.

This works perfectly, but now I want to test it!

Is there any way to identify the Invoke from a specific subscription? I ask this because it could be that in the background other things are executing certain commands that call the GetComments. I need to identify in my unit test that the Invoke is coming from my modified SearchText.