How can I tell if a Watch Later video has been successfully preloaded? Preloading of 'Watch Later' videos has been enabled, but I do not know how to tell when a video I am about to watch has been successfully pre-loaded by the YouTube app. As others have noted (Youtube Preload limit?) there is a limit to how much video Google will download to your device. I cannot see whether a particular video is preloaded or not. I have a good (but limited) mobile network, so preloaded or not pre-loaded both load quickly.

Can you see where the data is stored? Is there a widget or indicator I missed that tells me if the video will come from the storage or from the cell network before launching it?
I assume you're talking about the Youtube app. As far as i know videos are only preloaded when your phone is charging and connected to wifi. When a video is preloading it shows a little white arrow pointing down on the bottom right of the video (when viewing your watch it later list). When it has fully preloaded it shows a green circle with a white down arrow in it.

The files are stored in /sdcard/Android/data/com.google.android.youtube/files/ then there will be another folder (mine is seemingly random numbers). Within this there is one file per preloaded video. As far as i can tell they aren't able to be played with anything other than the youtube app.

This screenshot shows videos that have been preloaded: