That's why my suggestion is that it should be automatic but over time. I also do not want to do the tedious work of keep checking everything for every fight. Over time your characters will get to know them and show the numbers on screen.
If they hit with fire they learn if it is resistant to fire or not. They keep that knowledge with them forever so if the same type of another enemy shows up you just know what they are weak to. The circle not only should just show % but should also show if they are resistant to the damage you are doing after that. For example it should say %80 to hit, enemy resistant to fire. Below it.
In easier difficulties, they can just know anyways. No need to do anything.
I know this isn't extremely dnd but my reasoning for this is that from what I see in dnd games combat comes second and story, role play, info gathering comes first. In this game you are in combat QUITE a lot. Which I like because this is a strategy video game, making a QoL improvement like this will make combats smoother rather than "oh I should check this" or " let me look at the combat log 200th time because I forgot what goblins were resistant to"