According to my understanding of non-profit organizations--for one to be registered charity and become an actual non-profit organization, they must file with the state they reside in. Which means they would of received some type of document or license stating what they are and what they do.

If they don't have some type of proof, you can always check with your local BBB or your state registry on registered non-profit organization.

As well as a tip if you are doing this locally. Make sure when dealing with local charities to take care of them entirely. Charities have an outstanding voice in communities--which can hurt or benefit you in the long run.