check out Gina Ford's potty training book (you can get it from the book depository for around $10 including postage!)

I think the three things to look out for are:-

1. they are aware when they're doing a poo or pee
2. they can follow simple, clear instructions ("Archibald, please go pick up your red car")
3. they can basically dress and undress themself

but it's a little more invovled than that. but I think they're the main indicators

good luck!!!