The practical purpose of therapists is to ensure you have no mental condition which could be confused as Gender Dysphoria, and you are stable enough to make life decisions. The diagnosis for GD is fairly simple as a persistent need to affirm a gender different to you were assigned at birth, and it is basically you saying you fit this descriptor as there are no other clinical tests.
Eliminating all other possibilities is way more difficult, and my psychiatrist gave me a number of those questionnaires with cryptic questions they score. I think there were 11 different tests from memory. He sent me the quizzes after our first session, which I filled in and sent right back, and we had one more session to ensure I had been informed of the risks, was committed to proceed, and prepared to deal with any negative consequences. This, plus a letter from a psychologist, satisfied the WPATH requirements at the time.
At no time did either my psychologist or psychiatrist try to guide me in any direction, and this is how it should be. My psychologist, who was WPATH registered, claimed they diagnosed me with being Transgender, but she had no capacity to do so. At the time we were working with DSM-4, which had a diagnosis of Gender Identity Disorder, but it was already under review, and was dropped in DSM-5. Such a diagnosis opened the way for the disorder to be 'treated', or 'conversion therapy', which has been universally proven not to work, and to create disastrous outcomes.
It is important to note that the diagnosis for Gender Dysphoria is a voluntary statement, and it is not listed as a disorder. There is no psychologic treatment other than support, and the only accepted treatment towards sustained relief is medical. While psyches are often helpful in stressful processes, they often assume they can diagnose and treat transgender and we need to be wary of unnecessary and unhelpful methods. Some governments and institutions gate keep via psychs, and subject trans people to more distress than necessary, but if you are in those countries, you can be stuck.
Hugs,
Allie