Ultimately, you don't know, in many cases.... and we've reason to believe that in many cases where you should be, you aren't.
The game's event log is woefully incomplete and unhelpful in a lot of cases, and doesn't report dialogue events at all.
It's possible to see a dialogue advantage check, supposedly, in the intro when you're interacting with the brain - if you pass a check, you have an option to make a subsequent check at advantage, which it notes by making the dice green and displaying 'advantage' on the hover tooltip where it describes the skill and its modifiers before you roll. However, there is no indication that it actually DOES roll advantage there - we only see one die - and the event log doesn't record or report it, so we don't actually know if it is or not, and we cannot test or tell.
With that example, however, others have indicated that spells like 'friends', do not seem to be working as described or as intended, since they never present that 'advantage' notification in any dialogue.