The typical signs and symptoms of an eczematous patch are usually enough for an experienced physician to make the diagnosis. Supportive past and family history of the patients further aid him in diagnosis of eczema / dermatitis. In some cases however, the physician may need to take a small scraping from the eczema lesion to examine it under microscope for ruling out fungal infection. Skin biopsy may be required to rule out certain other skin diseases that mimic eczema / dermatitis.
Allergy tests may be required to determine the allergens or the irritants that are causative factors in a particular case.
Patch testing is also commonly done by physicians to pin-point what substances are capable of inducing eczematous reactions especially in cases of contact dermatitis.