Trivia: The only true black belts on the show were Leo Howard, Olivia Holt and Wayne Daglish.
Trivia: Leo Howard is in the Guinness Book of World Records for being the youngest person ever to direct a television episode. He was only 16 years old when he directed the episode titled "Fight at the Museum" in season 4.
Answer: It depends. If you watch the action scenes in slow motion, you can often tell if it's the actor or a stunt double. Leo Howard seems to do a lot of his own fight scenes and acrobatics while many of the fights and flips by other characters are done by doubles; although occasionally Olivia Holt will perform a simple flip or fight.
Both Leo Howard and Olivia Holt have black belts and completed their own stunts.