"Diagnosis" literally means, "through knowledge." On clinical grounds,
synesthesia is diagnosed when the perceptions are:
Involuntary, but elicited.
Synesthesia happens to people, and
the stimulus that sets it off is easily identified.
Projected--experienced not in the mind's eye but outside the
body. The only common exception is in those in whom synesthesia consists
only of colored letters and numbers.
Durable--a given synesthete's associations remaining the
same over a lifetime.
Generic--radially symmetric, scintillating, or wavy shapes:
disagreeable tastes; elementary tactile sensations. Synesthesia is
NEVER pictorial, elaborate, or a fully complete object.
Memorable. While comments like, "she had a green name," are
common, it is the sensation (green), and not the attached meaning (the
name) that is vividly remembered for decades.
Emotional and noetic--synesthetic experience is accompanied
by a sense of certitude, or even a "eureka" feeling. They seem not just
state of perception, but of knowledge.