I've sent several dogs to the Rainbow Bridge over the years, and yes, they've all told me when it's time. However, you have to be receptive to their wishes. We had a boxer, Duke, several years ago who got a very fast developing cancer, and one day, he came and stood in front of me, hung his head almost down to the floor, then looked up with very sad eyes. He was asking me to help him. We sent him to the Bridge that day. It was so sad, but we felt we followed Duke's wishes.
When your dog looks sad, doesn't want to do her usual activities, mopes around, goes off her feed, and looks miserable more than she looks happy/content, you'll know it's time.
Prayers for you as you decide when the time is right. It sounds like your girl is still having some good days.