How can I tell if I have dandruff? My scalp is very itchy, when I scratch it, moist flakes come off onto my hands as well as dry flakes.

Dandruff. Sounds like you have dandruff. Your description of intense itching and moist flakes are the underlying cause of your dandruff, which is dermatitis like seborrheic dermatitis. Psoriasis is also a possibility. Try otc dandruff shampoo. If no relief, check with your doctor.
Flakes. Since you are shedding flakes, chances are you have dandruff. You might try head and shoulders shampoo fei itchy scalps or selsun blue shampoo.