How will I know he is ready to potty train?

My son is 19 months old and I am in no way trying to rush him. BUT- he has gone poop in the potty several times. My in-laws put him on the potty the first time because he was having a hard time going and they figured it couldn't hurt to get the diaper out of the way so he wasn't pushing against something. Now if I see that 'look' on his face I ask him if he needs to go and he'll say yes and run to the potty (I just hold him on the adult toilet). Then he likes to flush and say "bye bye poopy". He has never gone pee in the potty, but he will stand up in the tub and start and stop peeing and say 'pee pee'. I realize he is a bit young yet....but I don't want to underestimate him either. He's really smart and understands a lot, he communicates incredibly well for his age. How will I know if he is ready? And how do I begin once he is?