Waterhouse, in Indiana, one carries only by (must issue) permit and "concealed is concealed;" there's no way that I know of for businesses to disarm their customers, no magic sign wording or dimensions. If you're carrying openly, they can ask you to leave.
Employers can make carrying a weapon on company business and property a termination offense; they can establish rules that do not allow carry on their property, so one must take care to not flout such rules. (ahem).
"Off-limits" places, like the combined court/city government/police building in Indianapolis or the House and Senate chambers in the Capitol building, usually have a sign listing what can't be carried in...right in front of the metal detector you must pass through to enter. The airport is the same way, though I would strongly caution against open carry.
If they want you disarmed here, they take steps to ensure you are. If there isn't a checkpoint, as far as the law's concerned it's more of a suggestion, a reminder to be discreet. "No guns" signs mean "No visible guns."
--Herself