How do I tell if transitioning is finished?

I have my levels for my game set up as small rooms and each room has it's own virtual camera. I know there is a way to check if a virtual camera has become active, but what I need is to know if transitioning has started or finished. Is there a way to do that? Or do I have to create a workaround?

 

I hadn't had a chance to try this until today. This is exactly what I needed. Thanks!