how do i know the difference between full movies and previews?

Your question has nothing to do with LW beta so please don't post questions there unless you are using the beta & have a comment about it's operation. I moved your thread.

Be aware that everytime you preview a file, it leave a copy of the file called PREVIEW - (filename).(vid extension) in your incomplete folder. So after downlding the file, arrange the incomplete folder by name or date & delete the preview files.
Personally I just drag & drop the video from the incomplete folder onto one of my video players/double-click it. I use VLC for most previewing since it handles incomplete files very well.

Once a file is 100% downlded, LW moves the file from the incomplete folder to the downld folder or "Shared" depending on what you named your downld folder.

Preview files will be the same size as the file was at that moment of downlding & will remain in the incomplete folder.
eg: If you previewed a video that has a total size of 700 MB but it was only 10% downlded, then the preview file size will be 70 MB.

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