How to determine if an event is already subscribed

In my .NET application I am subscribing to events from another class. The subscription is conditional. I am subscribing to events when the control is visible and de-subscribing it when it become invisible. However, in some conditions I do not want to de-subscribe the event even if the control is not visible as I want the result of an operation which is happening on a background thread.

Is there a way through which I can determine if a class has already subscribed to that event?

I know we can do it in the class which will raise that event by checking the event for null, but how do I do it in a class which will subscribe to that event?