Revealing mistake: Kim is supposed to be hanging from the tree by the ropes around her wrists, but you can see she is standing on a platform.
Kickin' It on Our Own - S2-E21
Revealing mistake: At the karate demo, the Black Dragon student never breaks the second board, the kid holding it drops it as the kick comes close.
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.