Gabby's Gold - S3-E8
Revealing mistake: When Kim and Gabby swing from the bar to kick the two villains, its obvious that Gabby kicked the guy in the knees, but he falls holding his chest. Plus Kim was a good 8-12 inches from her target and he fell anyway.
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.