this is a sure sign. a good idea is any time that your daughter seems interested in the potty, wants to sit on it, or anything, let her. it may seem like a huge waste of time, but you dont want to feel like you have to force her to sit on it either. i always have a pile of books for my son to look at while hes sitting on the potty.
it is a very good idea to have one of those little potty stools of her own so that she can touch the ground while sitting. this will give her a sense of security because she wont feel like shes going to fall! ive noticed my son would much rather sit on his and touch the floor than sit on the big potty.
my son is 22 months and is not potty trained by any means, but if i can catch him before he poops, and get him on the potty, he will go there. and he likes it. make a big deal about sitting on the potty, or going in general. you can pick predictible potty times like after naps or in the morning, before and after meals, before naps and bedtimes, and i think around 10-15 minutes after a meal is probably best.
remember this though; some kids can potty train early, when others just dont have the ability to hold it very long.
some signs that shes feeling like having to pee are getting quiet, getting restless, acting crazy that sort of thing.
potty training is something that i dont look forward to because it requires so much time - some kids have to go every 10 minutes! its all about what you are willing and have time to do. dont expect anything to be perfect.
a good idea is to get cloth diapers, they are better for training, and they let your daughter feel more wet more easily. it is however, in my opinion, a good idea then to watch out for having to poop. washing a peed in cloth diaper isnt as bad as a poopy one LOL.