What you did at the end can be a good approach tbh. Be curious and ask something about the post, or something in it that you're not sure of or familiar with, or if the OP hasn't added any body text or indications in the title and simply posted, what their opinion on it is, or if it's like a show/story/site/product, what's the premise or what's up with it and what they like about it.

Also if you don't want to focus it on the OP, in the absence of other comments, you can say as much like, "Do you (including anyone else reading) know/think [somethingsomething here]?"