How to know if that is eczema for sure?

In order to: be absolutely SURE see a qualified (Board Certified) DERMATOLOGIST! There are a LOT of skin disorders that may "mimic" eczema and there are a number of TYPES of eczema as well! Hope this is helpful! Dr Z

Atopic Dermatitis: Eczema is a common skin disorder that may not require a dermatologist to diagnose. It recurs characteristically on the neck and flexor creases of the arms,legs and scalp. Those with eczema unlike dermatitis from exposure to toxic metals or poison oak, have episodes beginning in childhood, a family history of atopy including asthma, sinusitis and food intolerances. It responds to topical steroids.