Potty training is so hard because it is so dependent on their personality. With our middle daughter the only thing that worked was to just switch to underwear and be prepared for accidents for a few days. She was completely good after a few days - we just had to stay outside or on the hard wood floor.
Knowing your child is ready is, knowing your child.
Ask yourself, does she show signs of being ready?
One good sign is her diaper is dry in the morning when you wake up. Another good sign is she can tell you she needs to use the restroom before she goes.
My youngest son (three years old) needed to move up a class in Montessori school but he had to be potty trained to do so. I had been trying pull-ups to no avail.
Finally, I did what one of the other moms stated and just put him in underwear and dealt with the accidents.
Number one took about 2 days and number 2 took almost two weeks but it worked. I don't think he liked the feeling of being wet/dirty.
Anytime he had an accident, he had to help me clean it up and I would explain to him that big boys use the toilet and that he is a big boy. Also, I read the book to him called "Everyone Poops" which has a boy but there are other books about potty training you can read to your little girl.