When your child is ready to start potty training and be done with wearing diapers they will usually start letting you know when they have a wet or poop diaper and will insist that it be changed.
That means that they recognize that they've went pee or poop and they don't like the feeling of the poop in their diaper or the wet diaper.
They will usually start tugging at the diaper as well even when they can't fully talk.
Your child might also start getting his/her own diaper and lay down for a diaper change or even try changing their own diaper.
When you start seeing those signs you should start introducing the child to the toilet.
You can start as young as 18 months old but usually it's better to wait till at least 2 years old to potty train your child since that's when there bladder muscles and motor skills are more developed enough to know when to go to the potty and they can hold in their urine better at 2 years old vs 18 months old.