AdRotate has no indicator if there is an advert or not for other plugins to use.
But, in AdRotate Pro you can use a Fallback group. Using that you can show another advert (or something) if there are no real adverts available.
For example you could stick an adsense advert in there. Or a sales banner encouraging people to advertise on your site. A “Your advert here, click for info” kind of thing.