Rene -
The answer to this is not a one-size-fits-all, unfortunately.
The differences are not just state to state. Federal, county, and city jurisdictions should be considered as most have laws specific to this subject - and they are all different. These laws may include a list of locations where a gun owner may not carry a firearm - generally referred to as "prohibited places." It can be surprising what is and is not listed, depending on your location.
Most jurisdictions allow private property owners the right to possess firearms on the property. They may require the property owner to post signs bearing specific language. As an example, Texas requires a sign reading "Pursuant to Section 30.05, Penal Code (criminal trespass), a person may not enter this property with a firearm." This sign must be in both English and Spanish. Other jurisdictions only require a "No Guns Allowed" sign, if anything.
If you are part of a government entity, contact the general counsel/legal department for your agency, department, division, etc. If you are a private organization operating on/within a government facility/property, you may not have a say in who can do what. It may already be a "prohibited property" or it may be an allowed carry location.
If you are a private organization operating on private property, my best recommendation is to start by Googling your state with the term "open carry laws." A number of public and private organizations will be listed as sources. Anything citing explicit regulations, public laws, statutes, etc. should be looked at. Keep in mind that private organizations supporting private gun ownership will likely have the most accurate information (outside of government sources). Typically, they will explain the laws in simpler language as they want to keep their constituency out of jail. When looking at these results, take into consideration the dates of the various posts on the subject. These laws may change regularly and a two year difference may greatly affect treatment by a state legislature.
As a reminder, "Open Carry" does not mean they can carry wherever they wish. It just means they can have an exposed firearm on their person. This generally includes a requirement that they are not creating a public disturbance.
Lastly, I am not a lawyer and the above should not be taken as legal advice. If you go through the above and are still uncertain, there should be people in your area who may have the answers you need. Oh, and don't call the police department. The vast majority of police officers are not familiar with property law and firearms.
Good luck!
Dave K.