You can never be too careful this time of year, as fox can sometimes be found in groundhog settes that otherwise have little or no evidence of fox occupation. While a fox in estrus will emit a stink, natal dens often barely smell at all -- a chemical change in the fox, and a natural protective defense after she whelps.
If you do come across a natal den, handle the kits as little as possible and put them back in the den. The mother fox will return very soon and move them, provided you clear out. You can rest assured she has dug several dens close by, as fox will often move their litters two or three times even if undisturbed -- a way to lose flea and parasite infestations, and also to reduce run marks that might lead a dog, coyote, bobcat or human to the site of the den. If a kit is harmed by a dog, humanely dispatch it and remove it from the area of the den after returning the unharmed kits to the den. Fox mortality is naturally higher than 50% in the first year, and the loss of one kit, while regrettable, will actually improve the odds of the remainder.
The kit, above, was an accidental run in at a groundhog sette about a dozen years ago. There were five kits inside, and this one was removed unharmed by the dog. She was scooted back inside the sette without a mark on her, and she tumbled back down the pipe. The sette was carefully repaired with a bit of scrap rug found in the hedge, and the cubs were later spotted several weeks later and just one field over.