Toenails worn too short?

When I first got my pup with her little needle teeth and razor sharp toenails, I just knew that one day I'd have to invest in a nail grinder. Well, she's five months now and that day has not come. In fact, her nails are short, short, short without any intervention on my part.

She will sometimes get the zoomies, running like a little rabbit inside and outside the house. Mostly, her running is outside when we play fetch. We start on the cement patio and then she hits the grass and goes after her toy. On average, she will cover 3-5 feet of cement before reaching the lawn during each throw. She's a leash puller, so her trips across the patio to potty probably add to the cement to toenail contact.

Do I need to be concerned that she will wear her nails down too short?

Her nails are black. Even as a pup with thin, sharp nails I could not tell where the nail ended and the quick began. How do I know if her nails are worn down too far? And if they do get too short, what should I do about it?