More than once I've "surprised" my players by having them getting jumped by enemies they couldn't see on the table, because I had unchecked the "Visible to Players" menu instead of hiding them behind the fog of war. The only way that I can see right now to tell if a token is player visible is by either clicking on the token and making sure the checkmark is in place, or by switching to "view as player".
What would be nice is if tokens not visible by players would have their colors desaturated until they are made visible, that would make it easier to notice this at a glance.