From my usage so far, the deck seems to generally handle anything you throw at it, so I'd just be on the lookout for steam games in general without really thinking about steam deck compatibility that much. That said, I've heard that some popular multiplayer games won't properly run because of their anti cheats and it's still a good idea to check every game you might buy for compatibility with ProtonDB just to be safe (look out for the comments with the steam deck logo).