Like it says; how do you know what a particular perk will do?
You press Q.
I think that the question was not exactly about “What skill does what” rather “What exactly does this skill”. Personally I would love to have a special button dedicated to some solid numbers like “How much can I repair with my Craftsmanship skill” or “How much does this perk increases my stealthiness”. This would be pretty handy for some math-lovers to say the least.