Keep an eye on it
What it can look like. A general, vet-reviewed example — signs vary and this is not a diagnosis. When in doubt, ask your vet. 
Itching & scratching in dogs
Clinical name: Pruritus
Frequent, restless scratching that the dog can't settle from — the hallmark of allergy.
Where it shows upface · paws · ears · armpits · groin
Typical patternFace, paws, ears, armpits and groin.
Linked allergy typesFood allergy · Environmental (atopy) · Flea allergy
Food vs environmentalYear-round itch leans food; seasonal itch leans environmental.
What you can do
Note when and where it happens, and what your dog ate or did beforehand.
Don't confuse it with
Other causes a vet will want to rule out: mange, dry skin, fleas.
This is educational information, not a diagnosis or treatment plan. Always see your veterinarian about your dog's health.
Related signs
Excessive lickingKeep an eye on it
Repetitive licking, especially of the paws and legs, often to the point of wet fur.
Chewing & biting at skinKeep an eye on it
Gnawing or nibbling at the skin, sometimes leaving damp or raw patches.
Face rubbingKeep an eye on it
Rubbing the face and muzzle on carpet, furniture or with the paws.