1. Long toe nails.
    Often it's men, sorry guys, I've seen a lot of feet doing acupuncture. The the lint from black socks get under the long toe nails and looks like ancient dirt, lodged in there for good or is it ancient dirt?
  2. When every other nail has a sliver of chipped polish. Maybe one without
    Just saying it happens. And I can completely not notice on my own feet.
  3. Fungus
    Admit it, that's the first step. The nail is thick and slightly yellowish. Get help. You do have a problem and you could be spreading it to others.
  4. When you have to expose your feet in public
    Barre class, swimming, life guard...
  5. When you are missing a toenail
    I said it. Did you cringe? Runners and basketball players and ballerinas know what talking about.