Main decisions in the game will elicit approval/disapproval from companions even if they are not present. It is sometimes difficult to tell because if a companion is already on maximum approval then the game will not display the reaction because effectively it hasn't changed. However, if you take the opposite choice then you will see disapproval.
In the Well of Sorrows you will see the approval/disapproval show for characters not present. For example, I'm pretty sure Iron Bull disapproved of me doing the maze puzzles to gain entry even though he wasn't in my party. Presumably if you just jump down and follow Cory's lackeys then he would show approval (although I have never done this so I can't say for sure). Really the only way to discover what is best for your walkthrough is to save, make the decision and watch the reaction display. If you really can't live with the result, then reload and do the opposite.
Not everyone will have a strong opinion about things though. So drinking from the Well or not may not elicit a reaction from them. I seem to recall that Sera did not show up as having strong feelings when I was in the Temple and she wasn't with me, but she sure made her feelings clear if I drank from the Well when we got back to Skyhold.