It matters if they want to "do it right" or just keep doing what they are doing. And why do they want to do this?

You would confirm with the State that nothing was ever submitted, and check the Feds not-for-profit listings. It's not that hard, really. Nearly everything is online. That will tell you what they submitted as.

Example:

A Gold Wing Motorcycle riders club is a privately-held LLC, not any sort of 501(c) entity. Surprise!

The BMW MOA tells everyone they are "nonprofit" but the IRS disqualified them decades ago, which made them a non-share-issuing Corporation. Surprise!

Why do this? Well, someone gets the bright idea that they can qualify for "donations" and that isn't true, most times, even if they become 501(c) something.

And your State controls a lot of this. In FL, it's the Agricultural Department that I had to deal with, for a Women's Club that supports their local library and provides pre-paid phone cards to military.