Hi,

'Legitimate' is kind of a loaded word. A non-profit can have all of the required papers filed, and still not be worthy of your interest.

If they are soliciting donations, you can ask for copies of their I9 documents (IRS Filing), their annual statements, bylaws, funds disclosures, etc. They may not be required or willing to provide them, but if they are not 'legitimate' most will go away.

You should also be able to visit their facilities and see for yourself just what they do and talk to the staff. Again, unwillingness to open their doors can be an indication of their 'legitimacy'.

Hope this helps.

Source: http://www.abusinessmentor.com