If pages are set to allow people to post on them, there will be a box that looks like the status update box, usually just below the cover picture/info section. I think it's normally on the left column, and you can post a comment or photo/video.
Sometimes it can be hard to see, it kind of blends in to the background.
If you find pages that don't have the box, it's because the page owners have set their page to not allow people to post on it. (I keep the rabbit rescue page I admin like this so I don't have to spend all my time deleting spam.)