How do you know when to transition your kid to 1 nap?

For my kids I did the transition when they were almost 2 years old. You would kinda know when your child is able to be active for much longer hours and taking two naps just doesn't seem that possible for him anymore. However if your kid is going to attend full day childcare in a childcare centre at 18 month, they only have 1 nap usually between 1 to 3pm.