From all the stuff I've read and all the potty training posts I've seen, expecting a toddler to tell you he has to go is the number one mistake parents make.
Do NOT expect him to tell you or to answer "Yes" if you ask until he has been fully trained for at least 6 months AND is over 3 years old. Even then, he will forget and have accidents. "I forgot" or "I didn't get there in time" accidents are common into grade school.
Take him often. If he is still learning, take him once an hour. Once he is well on his way to being trained, take him every 2 hours.
Do not ever ask him if he has to go. He will tell you "no" since he won't want to leave his toys. That's common and natural. Do not ask. Simply say "It's time to go potty".
When can you stop taking him? When you see him going down the hall all on his own, day after day, with no accidents. Until then, keep track of when he goes and get him there every 2 hours.
at 2:43 PM on Mar. 3, 2009