What is the way to determine traits in dialogue?

I think it's intentional that you only find out afterwards. As it's not very obvious from the dialogue which trait pair is going to be affected (especially when it's one you haven't discovered yet), the only way I found to do this was to save before initiating conversations, see what happens and then repeat with the desirable outcome (since once you know the traits, it's usually easy to tell which answer goes in which direction).

There is a list of the existing traits, but I have yet to find a list of the conversations that affect them (and I'm not sure I'd want to, as that'd be very spoilerific).