How to know you know

Someone asked a question in AskMetafilter (the best community answer service I know of) about how to learn the esoteric of currency exchange and trading. I thought the answer could be generalizable for all kinds of things: How to know you know. 

bonaldi: what’s the best way to learn that stuff? 

b1tr0t: Read books, google around, check wikipedia. When you start, it will look like most of what is said is reasonable. You are in good shape when you think that 90% of the stuff you read is BS.