How do I know when my toddler is full already? I don't want her to overeat

usually a toddler will know their own limit. My 2nd son (2yo ) he will EAT alot. I always thot that he doesn't know how to stop. But one day i actually find out that once he is full. he will reject food. No matter how yummy or tempting that chocolate is, once he is full, he WILL stop.