If it's a group and both of you are members of this group and not following each other, then by visiting his profile you will see his last activity and posts there.
If it's a page with a huge list of posts then your best shot is to use a tool to scrape the page's public data.
I found few by searching on google but can't vouch for any of them.
TryNinja and LoyceV are experienced with scraping data so maybe they can help! (mentioning their names so they get notified).