you need to just keep talking to them past where the bar seems full. and most npcs you can date you can always look up on the wiki if unsure
That is the first place I checked and even then even if I look at the NBC in question it doesn't tell me if I can be in a relationship with them or not
unless specified other wise i think you can.
And are there specific steps I need to take to start dating an NPC? After I reached Confidant
