In my state our CCW advocacy/lobbying group has done a lot of good work. One such victory is that we got the legislature to spell out very specifically what constitutes a valid "no CCW" sign. The sign's size, color, language, placement and symbology are all spelled out in great detail. For premises that have multiple entrances, every publicly accessible entrance must have such a sign. In "open air" venues the signs must be huge (3 feet by 4 feet, mounted at a certain height like 5 feet off the ground). Any business/entity that posts against CCW and does not make their signs like this has no legal standing for their anti-CCW action.
Do we go out of our way to notify businesses whose signs don't meet the code? No, we do not.
And that's the way those type of laws should be written, very specific.
But don't get me wrong I am very much opposed to gun free zones (except court rooms, police stations, etc) and personally I avoid places that don't allow me to carry.
However on the other hand, I also believe very strongly in a property owner's rights to control and govern his/her property as they wish. I also realize that no matter what steps the property owner takes they are most likely going to be sued in a situation like Nebraska and win or lose it is going to cost them a small fortune in legal fees.