If you aren't sure, it is usually easiest to ask the person directly if they need help. If they want help, they can ask, or they might not need help yet. That is usually the easiest way. If she was about to walk into something dangerous, though (something stick out that the dog missed), then as soon as possible would probably be the best.

dont be afraid to ask i get asked all the time and if i dont need help or they are doing something wrong i let them know so they dont make the same mistake twice