How can i tell the difference between dandruff and seborrheic dermatitis? is there any harm in using sd shampoo if it's only dandruff?

Pretty much the same: Dandruff (seborrhea) is common. Shampoos with zinc, salicylic acid, selenium, or coal tar are sold in many stores. Nizoral shampoo (contains anti-fungal) is also available. Doctors may also recommend cortisone or kenalog (triamcinolone) sprays. Prevent dryness (don't wash hair too often). A primary doctor or dermatologist can look for psoriasis, eczema, or fungal infections if unusual or worrisome scalp symptoms.