Has an event handler already been added?

I recently came to a similar situation where I needed to register a handler for an event only once. I found that you can safely unregister first, and then register again, even if the handler is not registered at all:

myClass.MyEvent -= MyHandler;
myClass.MyEvent += MyHandler;

Note that doing this every time you register your handler will ensure that your handler is registered only once. Sounds like a pretty good practice to me :)

From outside the defining class, as @Telos mentions, you can only use EventHandler on the left-hand side of a += or a -=. So, if you have the ability to modify the defining class, you could provide a method to perform the check by checking if the event handler is null - if so, then no event handler has been added. If not, then maybe and you can loop through the values in Delegate.GetInvocationList. If one is equal to the delegate that you want to add as event handler, then you know it's there.

public bool IsEventHandlerRegistered(Delegate prospectiveHandler)
{   
    if ( this.EventHandler != null )
    {
        foreach ( Delegate existingHandler in this.EventHandler.GetInvocationList() )
        {
            if ( existingHandler == prospectiveHandler )
            {
                return true;
            }
        }
    }
    return false;
}

And this could easily be modified to become "add the handler if it's not there". If you don't have access to the innards of the class that's exposing the event, you may need to explore -= and +=, as suggested by @Lou Franco.

However, you may be better off reexamining the way you're commissioning and decommissioning these objects, to see if you can't find a way to track this information yourself.