There are a couple of different ways to check this. The easiest is to use a probe thermometer - they're not expensive and they're a serious game-changer. You want the internal temperature to be 75C/165F, in the thickest part of the leg, near the bone. Without a thermometer, if you cut into the chicken you're looking for the juice to run clear - if it's not cooked enough, it'll be pink and cloudy. You can also cut into the meat itself if you're really unsure to check - around the leg near the bone is the part that will take the longest to cook. It's tricky because some of the meat around that area will remain a little pinkish so don't be put off if you see that. You want the meat to feel firm - if it's super soft and rubbery you'll know it's undercooked.