To identify if a vid is Stream Only (not available for download) you will need to visit the vid page where you can find specific details before your purchase. Below the vid, it will say "Steam Only" to the right of the resolution.
To identify if a Membership is Stream Only, click on "Get My XX Vids" under the MV Star's profile picture and you will see a pop-up box that says "Get instant access to XX Stream Only vids plus exclusive and new vids added during your membership." If it does not specify "Stream Only" it means the vids included in the Membership are available to download.
Please note, MV Stars have the ability and discretion to change their vids and/or Membership to stream-only or downloadable at any time.