Just head to My Games & Apps, and choose to Sort by Size. Check the entry for Halo 5 - if it's greater than 40GB, you're OK. If it's smaller than 40GB, you've got a dud file, I'm afraid. In that case, hit the Menu button, delete that entry, then go the Xbox Store entry for the game - it'll have a big button that says Install. That'll net you the proper version of the game. Time saved (sort of).
Here is what your preload should look like!