Start with where the dog is itching
Fleas announce themselves through the dog's behavior before most owners ever see an insect. Persistent scratching, chewing, and licking concentrated at the base of the tail, the lower back, the belly, and the inner thighs are the classic pattern. A dog who suddenly cannot settle, who spins to bite at his back end, or who scratches through the night is often hosting fleas even when a quick glance at the coat shows nothing. Restlessness in a normally settled dog is a sign worth taking literally.
Flea dirt: the evidence that gives them away
Adult fleas are fast, small, and expert at vanishing into a coat, but they leave evidence behind. Part the fur and look for black, gritty specks resembling ground pepper, especially along the back and tail base. That is flea dirt, which is digested blood passed by feeding fleas. The home test is simple and reliable: comb specks onto damp white paper or tissue and wait a moment. Ordinary dirt stays grey or brown and gritty; flea dirt dissolves into a reddish-brown smear because of the blood in it. Finding flea dirt means fleas are feeding on your dog now or have done so very recently, whether or not you catch a live one.
What an established infestation does to skin and coat
Left running, an infestation marks the dog. Hair thins or breaks over the lower back and tail base from constant chewing, skin becomes red and scabbed, and raw, moist patches known as hot spots can flare where scratching is relentless. Some dogs react strongly to flea saliva itself, so even a small number of bites produces disproportionate misery, broken skin, and secondary infection. In a puppy, a small dog, or a dog already unwell, a heavy flea burden can drain enough blood to cause weakness and pale gums, which is a veterinary problem, not just an itching problem. You may also spot rice-like segments near the dog's rear or in bedding, a sign of a tapeworm that fleas commonly carry between hosts.
Check the household, not just the dog
Most of a flea population is not on the dog at all. Eggs and larvae live in carpets, bedding, and the cracks of floors, which is why an infested dog often means fleas are also found in the dog's sleeping spots and why other pets in the home may be scratching too. Finding specks in the dog bed, or bites appearing around your own ankles, completes the picture. This is also why treating only the visible dog, once, so often fails: the environment keeps hatching replacements for weeks.
When the vet should be involved
Call the veterinarian if the skin is raw, infected, or weeping, if your dog is a puppy, senior, pregnant, or unwell, if gums look pale, or if scratching continues despite your prevention routine, because that pattern can mean the product is being applied incorrectly, the infestation is environmental, or something else entirely is driving the itch. Bring your timeline: when scratching started, where it is worst, and what prevention has been used and when. Recording prevention dates and skin notes in KinsLe's Pet Passport afterward keeps the schedule visible, so the next lapse is caught on paper before it is caught by the dog.