How to check opinion between any two arbitrary people?
This feature has been made a bit obscure, but it is possible. Go to the character's screen then right click on their portrait. Now two buttons will appear, one with a head and one that is for diplomacy. Right click on the head button. Now three more buttons appear, the bottom one is for checking character's relations. You will then get the usual list of people, but opinions will be of the character you selected rather than of yourself.