I refuse to believe that there isn't. I am sure its not magical I am sure there is some reasonable rational way. Medical professionals must know. I sincerely doubt they just start handing out drugs and hormones to just anybody claiming to be trans. How do they determine who is eligible and who isn't? What are the criteria?
Could your recent post be a start? If someone didn't share those feelings?
I rather like Gendermutt's therapist's quote: "I also go back to early in my own journey I started some 2 and a half years ago when going to a very good gender therapist who has no agenda. What he said was that part of being TS, a big part of what defines a TS is not how much you like or want to be feminine, but how much you dislike being male or masculine. If you are ok with being a guy, regardless of how much you like or want to be feminine, then it is not a correct path to take to transition."
I heard something similar many years before it kind of stuck with me.
Granted there may not be just one predictor I am sure there are a few that can eliminate a person form the running or at the very least reduce or increase chances. While as you pointed out that GDs and CDs have very many similar behaviors there are probably very subtle differences that can be indicators.
Also if there is no way to predict wouldn't this negate what I have heard from many Trans, "you just don't wake up one morning a decide to be a woman" Would that not show that some knowledge of really being a woman beforehand? And that it is a long period of denial reaching critical mass? That prior knowledge being denied would be an indicator.
This would be a very productive line of enquiry. I can't be the first