How do i know a release date on a preorder item

After purchasing a preordered movie, I assumed I would be able to watch it right away

No; pre-order is available for titles that haven’t been released on streaming yet. Their availability date is in the future. It means that you’ll have to wait some time before you will be able to watch it, depending on future release date as set by the supplying studio. But you will be notified by email when it is available, and waiting for you in your Library.

however now I’m unsure when I’ll be able to watch it

If the supplying studio has told Apple what the release date will be, then it will be mentioned with that pre-order, and on that movie’s Details page. But it might be ‘to be determined’ later, without any known date initially. If they tell Apple at a later time (closer to release date) what the actual release date will be, then the pages for that title will be updated with that new date. It is also possible that the supplying studio chooses to reschedule by changing the release date after initial publication of a different release date.

View or cancel pre-orders for apps, movies, music, or books - Apple Support