If you go to your liege's council screen (the last tab on the right), you'll see a bunch of clickable buttons showing each position. Just click one. Be careful, it's a confirmation click and it takes some time before you can change again. You do have to be on your liege's council to actually pick a position.
Also, no, you can only guess based off of their stats as the game doesn't tell you what they will be. More detailed information is in the wiki but it's basically a "points" system. Traits give certain positions points, as does the current opinion of you, and things such as friendship/close blood relative also give points.