Answer first
Dandruff usually shows as larger, slightly oily or yellowish flakes with itch, often linked to yeast on an oily scalp, while a dry scalp sheds fine white flakes and feels tight. Dandruff tends to improve with regular washing and an anti-dandruff shampoo; dry scalp improves with gentler washing and less hot water.
Dandruff signs
Oily scalp, larger flakes, itch, sometimes redness along the hairline or eyebrows.
Dry scalp signs
Small white flakes, tightness, worse with air-conditioning or hot showers.
When it's something else
Thick silvery plaques, bleeding or spreading rash may be psoriasis or dermatitis; see a doctor.
Questions
Should I wash less if I have dandruff?
Usually no; regular washing helps control oil and flakes.
Can a scalp treatment help?
A professional scalp cleanse and analysis can show what type you have.