More to the point it's not available as a diagnostic test.
The research that's been done as far as I know has been based on fmri which can track neural activity by detection of increased blood flow to specific areas. It turns out, for instance that in cis gender people orgasm occurred in different areas if the brain for women vs men (as far as I know this work hasn't been done with trans populations yet).
In the papers I read about trans brains found that both mtf and ftm participants looked more like their cis gender counterparts with a higher correlation for FTMs than MTFs.
Like you, I'd love to be able to have this bit of data (to buttress my confidence in proceeding to GCS). However it's simply not available. I feel there are enough other ways for me to be certain. HRT has vastly improved my life, from sexual response to creativity to my ability to relate to others. I've tried going off of estrogen and it was a horrible experience.
All of my questions and second guessing went away over time. I'm at 10 months on hrt now and couldn't be more certain that I want to transition. Best if all I could imagine a way forward that didn't include GCS. I would be OK and that's oddly reassuring for me; I guess because surgery no longer feels like an emergency, rather it's the better path forward, a choice I'm comfortable with.