An organization is a publicly supported charity if it meets one of two tests:
The organization receives a substantial part of its support in the form of contributions from publicly supported organizations, governmental units, and/or the general public.
Example: A human service organization whose revenue is generated through widespread public fundraising campaigns, federated fundraising drives, or government grants is a publicly supported charity.
The organization receives no more than one-third of its support from gross investment income and more than one-third of its support from contributions, membership fees, and gross receipts from activities related to its exempt functions:
Examples: A membership-fee organization, such as parent-teacher organization, or an arts group with box office revenue is a publicly supported charity.
Publication 557, Tax-Exempt Status for Your Organization