For me to trust someone, they would have to:
a) make a lot of money trading, and
b) not make much money telling other people how to trade
A successful trader who has written a book might fill this bill, but only if it isn't a book that's a thinly disguised advertisement for their other services. You don't usually make much money from writing a trading book.
In other words, if they also had some fancy web site inviting me to join for a high fee, I would not trust them.
This is not to say there is nobody out there charging hundreds a month who is not legitimate and successful. Probably some of them are. But there is no way of sorting that out.
Contrary to what some people write on ET, I think there are those who want to share their knowledge.
In addition, when I do read anyone's book, I look for them to discuss how to avoid losing money. I think this is actually more important than making money.
