Most advisers I've seen have some sort of experience in their respective fields, either as a CEO of a company that utilizes the blockchain in their projects, or as a board member or CTO. Their knowledge and experience can be of great value to a blockchain/cryptoproject, on multiple levels: they may offer financial advice, organisatory advice, technical advice or help in networking and setting up a brand.
I think most of them are introduced to projects via networking, conferences, summits, meetings, you name it. Sometimes advisors may offer to provide services if they really believe in a project. I assume that, them being an official team member, most advisors get paid using the capital raised from investors / ICO.
Most of these advisors have LinkedIn profiles, so you can check their resume and past experience. Then it's up to you to decide whether they are trustworthy or not. Do your due diligence.