The problem with this is that we can't know simply because, somewhat ironically, we can't read other people's minds!
My opinion is that once self-proclaimed psychics start using their 'powers' as their source of income (they run psychic/healing businesses etc.) then it must become obvious to them that what they're doing isn't real as they hone their practises to whatever makes the most money.
Celebrity psychics are an extreme case of this. Whether they still have some genuine underlying belief that they have some ability who knows but they surely must be fully aware that what they do isn't real.
When celebrity TV psychics issue 'love cards' to members of their audience before the show asking them to fill in the details of the person they hope to 'come through' and then these deceased people miraculously do come though during the show, they are clearly involved in the trickery.
Although I'm sure there are plenty of people who genuinely believe that they have some psychic powers, I'm not too sure how confident they are that these powers are truly genuine.
Whenever they come on here proclaiming their powers/abilities and we ask them to participate in testing they never agree. That suggests to me that no matter how much such people want to believe, deep down they probably know it's all false. But the belief is more precious to them than the truth so they avoid confronting doubt.