You  don't know when your child is ready.  How do they know when they are ready? They don't.   You have to introduce potty training to them.  What  a ridiculous question. A child does not wake up one day and say "I think I need a potty today' They learn by their parents! It is a slow process, but can only be achieved by parent training.  (Or is it something else that should be the responsilbilty of childcare, nursery, or even their school).