Sure, some of things can be "tells," especially in combination. However, I submit to you that when you spot a cop, I believe it's much more about they way he carries himself and the way he is groomed. "You look like a cop" is a stereotype that exists for a reason and it's more common among the younger guys. It's not about the way they dress, but that can certainly add to it.
I have never been accused of looking like a cop, either in or out of uniform.