Hi Parker,
Most therapists will probably not make a diagnosis for a while, I would think,. Mine didn't. They obviously are going to want to know *why* you are seeking therapy, and if gender is on your mind that will come up in discussion pretty quickly. In my case I had a cross-dressing history and I was starting to go out in public dressed, so it came up pretty fast. I suspect diagnosing dysphoria varies greatly from therapist to therapist. It is more art than science, unfortunately. FWIW, this the American Psychiatric Association's summary of the DSM-5 criteria:
Diagnosis
The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) provides for one overarching diagnosis of gender dysphoria with separate specific criteria for children and for adolescents and adults.
In adolescents and adults gender dysphoria diagnosis involves a difference between one’s experienced/expressed gender and assigned gender, and significant distress or problems functioning. It lasts at least six months and is shown by at least two of the following:
A marked incongruence between one’s experienced/expressed gender and primary and/or secondary sex characteristics
A strong desire to be rid of one’s primary and/or secondary sex characteristics
A strong desire for the primary and/or secondary sex characteristics of the other gender
A strong desire to be of the other gender
A strong desire to be treated as the other gender
A strong conviction that one has the typical feelings and reactions of the other gender
I don't know if this is the actual language of the DSM-5. My therapist read the criteria to me at one point and I recall some language about a 3rd gender as well as the "other gender." But I could be misremembering. The bit about significant distress is important. If you were fully comfortable with your gender presentation, even if it is completely trans, you technically would not have dysphoria. And as you can see, there is significant subjectivity in the criteria. "Strong" and "typical" are not crisply defined.
I would guess any therapist would want to probe your self-perception of your gender identity, any concrete steps you might be taking to realize it, and what sort of emotions and/or problems it is causing. I have to say I really value my therapist. They can be wonderful relationships, in a provider/patient sort of way. I hope you can find one that's a good match for you.