But what about effects like Derren asks someone on stage and able to read them and tell what they do for a living, that kind of thing. I suppose that is reading isn't it.
My first suggestion. . . stop watching Derren (or any of the Tv guys) and seeing their shows as a shopping list.
As to your question. . . it takes years of study & practice + showmanship (experience) to make it look & feel the same as you see with Derren or other skilled Mentalists. If you really want to learn it take all the psychology and communications courses you can find and you'll have a decent foundation; study the books & videos mentioned including sources like TRADECRAFT, though it's technically a Cold Reading text it has a ton of statistical points that lend themselves well to this kind of thing.
Start at the beginning, learn the basics and in time you will find the more advanced methodologies. Understand that it is not 100% sure fire stuff and not everyone gets away with it, it requires practice & patience but especially practice.