With MapKit there's
MKMapRectContainsPoint(self.mkMapView.visibleMapRect, MKMapPointForCoordinate(coordinate));
and I need to do something similar with a Skobbler map instance.
Here's a solution that works pretty well:
CGPoint point = [self.skobblerMapView pointForCoordinate:yourCoordinate];
BOOL containsCoordinate = CGRectContainsPoint(self.skobblerMapView.bounds, point);