I know it's not an exact science, but I generally start with whether or not that entity has taxing authority. If you look at your tax bill, you'll see the county, city, schools; those constitute the bulk of our public funds. In one case we had a request from the local Water and Conservation District, and, surprising to many of us, they were a line item on our tax bills and the state office of the US Department of Agriculture confirmed that they used public funds.
In the absence of such a taxing authority I'd ask them to get some documentation from the overall entity that governs them(e.g., if this is a public airport, which legal entity is responsible for it?).
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I hear and I forget. I see and I remember. I do and I understand.--Confucius