How to identify a generic type at runtime?

public class Envelope
{

public string MessageId {get;}

public abstract Stream GetStream() 

}

public class Envelope<T> : Envelope
{
public T Body {get;}
}

public class MessageBus
{

public void Send(Envelope message)
{
sender.Dispatch(message);
}

}

public class MessageSender
{

public void Dispatch(Envelope message)
{

// THE PROBLEM IS HERE!!!!!
// HOW DO I IDENTIFY THE TYPE OF BODY? 
// On Envelope (base class) the property Body is missing and i can't get the type 
// to try create instance of change type .. and i need to pass body as parameter to 
// another method
}
}