Do some research first, have they been tested by others and proven themselves? Or did they just write a few books and make a name for themselves? Usually you will find many who have never taken such steps to prove their claims. Others actually get defensive if you dare question them. Still others are running websites and businesses taking money from people who fall for their claims.
The problem is often you can't find anything other than their own claims supporting them. Yet every day people continue to deal with unproven psychics and mediums. You are left on your own to prove or disprove their claims.
So do it. Before you accept them make them prove themselves to you. If you are considering consulting with anyone, go slow. Ask a few questions of them, but make them questions you alreasdy know the answers to, and something no body else can be expected to know. Then see how the responses go.
First put on your skeptic's hat; approach them as a skeptic would, not someone who believes they have any ability at all. Consider how they answered your questions....Are they vague or very defined? Did they actually answer the question or skirt the issue? Can you think of more than one interpretation to their response? Did they become defensive in their response? Or might they have completely ignored certain aspects of the questions? And even worse did they actually challenge you for doubting their abilities?
Their reaction to your line of questioning can reveal much about their claims. After all, why would anyone who has a proven ability have to defend themselves when all you are trying to do is verify somethey themselves claim? Unless of course they are not as sure of themselves as they want you to believe. And right there is the answer to their credibility, and whether or not you are well advised to put any stock in them.