How do I know if I even have photos on MY PHOTO STREAM?

You will only have photos in My Photo Stream, if My Photo Stream has been enabled on one of your devices during the last thirty days. On devices, where My Photo Stream has been enabled, it has uploaded the recently imported photos from the device into the Stream and kept them there for at most thirty days, so other devices could download them. The stream did not remove photos from the device where they came from, the stream created new copies of the photos.

If you have not been using My Photo Stream, there is nothing to do, for you even if you accidentally had turned it on some time ago, it will not have removed any photos from your devices, as it has been syncing new copies.

If you have been using iCloud Photos on your Mac, you could not have enabled My Photo Stream; on a Mac these two services are mutually exclusive. If have not been using iCloud Photos, you may want tocheck the Photos > Settings > iCloud, if My Photo Stream is enabled there.If it is not shown there or not enabled, you have not been using it.