When I buy books or courses about investments, how can I make sure that I am learning from legitimate educators rather than charlatans? What are some basic ways I can use to determine legitimacy?

I have always suspected that most self-proclaimed "stock market gurus" are charlatans because:

1. They don't provide statistical evidence of the validity of the techniques they teach.
2. They don't show you their audited trades, profits and losses.
3. They may make more money from selling "educational materials" than from the application of their investment methods.
4. The educational material they provide may be full of self-promotion, and they always attempt to up-sell (e.g. "buy my $3000 course to learn more!").

Points (1) and (4) are easy to check — simply read their book or do their course. Point (2) is not always valid because some people value their privacy. Point (3) is very difficult to verify. Are there other tell-tale signs of a fake investment guru?

Due to the perceived amount of charlatans who are authoring investment books, I have resorted to reading university textbooks. Books written by university professors are presumably less likely to suffer from the flaws I mentioned above, because university textbook authors have a different motive for writing books. If I now want to read "mass-market" investment books (e.g. those in the Wiley Trading Series), how do I filter out those written by charlatans?