There is no method on UICollectionView to give you this so here's what I do (iPad, fixed Portrait mode with a collection view subview smaller than the screen, so you probably want to be a bit more clever with the CGPoint);
// Get path for cell in centre
UIWindow *mainWindow = [[UIApplication sharedApplication] keyWindow];
CGPoint pointInScreenCoords = CGPointMake(384, 512);
CGPoint pointInWindowCoords = [mainWindow convertPoint:pointInScreenCoords fromWindow:nil];
CGPoint pointInViewCoords = [self.collectionView convertPoint:pointInWindowCoords fromView:mainWindow];
NSIndexPath* centreIndexPath = [self.collectionView indexPathForItemAtPoint:pointInViewCoords];
Depending on how often you require this to update you can put it in the scroll view delegate's didScroll method, didEndScrolling or just call when required.