The only fields completely invisible to the players are GM Notes (tokens, handouts, characters), and the Notes/Bio (handouts, characters) of things not visible in a player's journal. Even if they can't select the token, they can get the value with and If the information is stored on a character's attribute, they can get it with Name|attribute name} (and if there's a token linked to the character, they can also use selected/target).

That said, any player can get the bar's value with something like and whether they can edit the bar or not, whether they can see the bar or not. Brian said: On the Player Permissions tab of the Edit Token dialog, you can give them the ability to see the bar, but not edit it.