In my opinion, you should be learning about codependency and working on your own healing and not looking at your ex mates Facebook. It obviously upset you. And as you say he doesn't know you can see it...maybe he does and that was for your benefit. But I doubt it. All it is going to do for you to "check up on him" through social media or any other way, is going to hurt you. Move on with your life. Heal yourself and forget about what he is or isn't doing. It no longer concerns you.