I didn't say for everything and in all cases, but there are cases where you can't tell. GTA V and Battlefront both can produce screenshots with tons of detail (including foliage) that are near imperceptible from reality.
Where games struggle today is hair, skin, and soft fabrics, but there are already good implementations for those that should show up in the next generation game engines.
This will always been an iterative process, but there are times today (and I honestly feel like this just became the case within the past year or so) where games look like reality.
Clothes is one weak point with real time CG today. Another is grass, trees.
There is a reason there is no green in those pictures, and they try to only draw people in rigid suits.