I'm trying to get a careful Groudon in Omega ruby and I keep getting other natures. I already have a Pokemon with careful nature + Synchronize (Ralts) and I want to know if there is any way I can see a Pokemon's nature during a battle, like how you can see a Pokemon's ability using trace or an item using frisk.
No,you cannot.
You can only see after the battle.
Source:experience and this.