[JillnChris]

The first signs are when they are able to say "pee" or "poo" and are able to give you signs of when they are going or need to go.
When DD started doing this about 4 months ago I ran out and bought her a little potty chair. She loved it at first and always wanted to sit on it, she was successful a few times but then the novelty sort of wore off and now she doesn't care much for it at all. I still ask her a couple of times a week if she wants to sit on it but she will only sit for 2 seconds, not long enough to actually do anything. I have tried bribing her with stickers but she is still too young to really get that concept.
I think that learning to say the words and to tell you after they went or when they need to go is all a part of the process. If they know how to do that then you can just follow they're lead and when they're ready they're ready. It is not uncommon for boys to PT between 3 and even 4 years of age.
I think that if a parent wanted to put A LOT of time and effort into PT their child might do it sooner but for me I would just rather wait until she is ready. I hate cleaning messes!

Here is a great little checklist.
Potty training readiness checklist (age 2) | ParentCenter