How to determine when a custom UICollectionViewCell is 100% on the screen

This will return an Array of IndexPaths for the fully visible cells:

func fullyVisibleCells(_ inCollectionView: UICollectionView) -> [IndexPath] {

    var returnCells = [IndexPath]()

    var vCells = inCollectionView.visibleCells
    vCells = vCells.filter({ cell -> Bool in
        let cellRect = inCollectionView.convert(cell.frame, to: inCollectionView.superview)
        return inCollectionView.frame.contains(cellRect) 
    })

    vCells.forEach({
        if let pth = inCollectionView.indexPath(for: $0) {
            returnCells.append(pth)
        }
    })

    return returnCells

}

@IBAction func test(_ sender: Any) {

    let visCells = fullyVisibleCells(self.collectionView)
    print(visCells)

}

You can filter your visibleCells array to check if the frame of your cell is included in the frame of your collectionView:

    var visibleCells = self.collectionView?.visibleCells
    visibleCells = visibleCells?.filter({ cell -> Bool in
        return self.collectionView?.frame.contains(cell.frame) ?? false
    })
    print (visibleCells)

Here's an extension for it, based don DonMag's answer:

extension UICollectionView {
    var fullyVisibleCells : [UICollectionViewCell] {
        return self.visibleCells.filter { cell in
            let cellRect = self.convert(cell.frame, to: self.superview)
            return self.frame.contains(cellRect) }
    }
}