He stays dry for several hours at a time. He can hold back his urine for a minute or two even on a full bladder. He has learned the family's basic bathroom vocabulary and uses it correctly. He complains about wet or dirty diapers. He becomes curious about the toilet and asks for his own special potty.
Intelligence has nothing to do with it, but a lot depends on temperament.
Like all bathroom matters, toilet training is a very individual business, but you can look for signs of readiness around the second birthday. How you teach toilet training will depend on when you start. Success at this age usually comes from a combination of close parental attention and intensive behavior modification including frequent trips to the bathroom, enforced potty sits, repeated reminders, lavish praise, and regular rewards. In the last 25 years, professional opinion in the United States has come to discourage early toilet training in favor of a more developmental approach that lets a toddler's own drive for autonomy do more of the work. The goal is to get the child training herself.