Open Settings, General, Storage & iCloud Usage and see how much space is available. You can also select Manage Storage to see which apps are using the space. Then open the iTunes Store app, go to Movies, pick a large HD movie (like Batman vs. Superman) that is larger than the available free space, then click the rent button, confirm and wait. You will get an error message about not enough storage. As long as you don't have enough space, you won't be charged for the rental. If however you do have enough space, enjoy the movie! At the same time your rental fails, your device will have cleaned up a bunch of space in an attempt to make room for your rental. Check the available space again, and repeat the attempted rental process until it no longer frees up space (or you have enough to download the movie).