In principle, you should see that you will roll with Advantage when hovering over the dialogue option. There are a couple of occasions where you can see this, but I think they are all non-spell related.
After you have selected a dialogue option involving a skill check, when you are shown the roll target, you can hover over the target number to see the details.
Then, when you roll the dice, you only see one result. I suppose the game shows the better dice result only.
A spell like Thaumaturgy half works : when looking at an Intimidation dialogue option, it doesn't show you Advantage. When looking at the roll target, it informs you that you have Advantage indeed, as you should.
Friends doesn't work, as far as I could tell : when looking at a Charisma dialogue option, it doesn't show you Advantage. When looking at the roll target, it doesn't show you Advantage.
