While each UI is a bit unique (predictably, the iOS app foregoes any purchase options due to Apple's insistence on a 30 percent cut) one way to keep track of how a particular movie is accessible are via several small icons in the corner of its box art, showing a kiosk for discs, play button for subscriptions, ticket for purchase, and so on.
ENGADGET: More Redbox Instant details revealed at CES; Vizio, LG and Google TV clients coming