If you need to delete data from your idevices completely, use an ios data eraser tool, I think that is helpful.
You can erase all data without possibility to recover them, but remember to backup them first.
There's a free way to check whether the data still on your iPhone. Just Google "Safewiepr for iOS", it's not a totally easy to use app, but the iPhone data checking function is free. Safe and useful. it can help you erase all data from iPhone,iPad and iPod, like messages, contacts, photos and more..