You can't verify anything, unless they give you their tax returns.

You can do revenue share deals, where gurus are paid a percentage of profits they generate.

Tip: True Gurus (upper 'G', rather than lower 'g') are busy generating cash. You'll rarely meet them + hardly anyone knows them.

To meet real Gurus, traffic places like Meetup Groups with very tight topics, which relate to what you're trying to accomplish.

When you meet a real Guru, likely you'll know it by the way they talk about their area of expertise... which you'll likely have to tease out of them.

Anyone making massive cash... tends to keep their techniques to themselves.

Use Glassdoor.com

You cannot. You never know about it. In some countries it's even illegal to ask for another one's private net worth. In Sweden every salary has to be openly published. Move to Sweden!