How is Aquagenic urticaria diagnosed?

A water test is done by a professional medical facility such as York Hospital, Hershey Hospital, John Hopkins, and many more.

A diagnosis of Aquagenic Urticaria may involve the following tests and examinations:

Physical examination of the skin and medical history evaluation
Skin allergy test
Radioallergosorbent (RAS) test; to determine the specific allergy-causing substance
Water challenge test, which is a specific test undertaken for Aquagenic Urticaria
Tests to exclude other forms of physical/inducible urticaria
Blood test to detect various antibodies against allergens including:
Complete blood count (CBC) including eosinophil count since, there may be increased number of eosinophils (peripheral eosinophilia)
Antinuclear antibody (ANA) test
Erythrocyte sedimentation rate (ESR) test
Immunoglobulin E (IgE) levels in blood
Serum level of complement
Serum C-reactive protein blood test
Tests to rule-out other underlying autoimmune conditions and infections (if necessary)
Skin biopsy (if necessary): A skin biopsy is performed and sent to a laboratory for a pathological examination. The pathologist examines the biopsy under a microscope. After putting together clinical findings, special studies on tissues (if needed) and with microscope findings, the pathologist arrives at a definitive diagnosis
A differential diagnosis may be considered to eliminate certain skin conditions, prior to arriving at a definitive diagnosis

Skin tests can be done and sometimes the doctors can build up enough data so just diagnose you without any tests. I mean, with the symptoms, in my case there was nothing else it could be. Aquagenic Urticaria was our only answer.

Allergy test from an allergist along with a blood test.