Fair questions. It is hard to measure in a quantitative sense. It is more like what the Supreme Court justice said about pornography. "I can't tell you what pornography is, but I know what it is when I see it."

There is no genetic test for this since it has more to do with how the brain has formed while in the womb. It is something you'd have to observe in a child that is young and see their patterns over a lifetime. Watch how they manage conflict, what types of play they prefer, whether they are more into skill-based games or relationship-based games, see if they make better nurturers or providers, and natural mannerisms and affect, etc.

As for TGs, generally, that would emerge more in adolescence when sexuality emerges (reemerges if you buy into Freud). Transsexualism tends to emerge more during or before the "latency" stage of childhood development.

We have to distinguish between innate stuff and more cultural stuff. TS and TG have differing philosophies here. TSs know that their nature and "core" is the "other" already. So they'd want to fix the body to match who they already are, and tend to let the cultural side fall into place naturally. TGs don't always see their identity as being at odds with their bodies and might never change that ("I can still be a woman with a penis since man and woman are arbitrary social constructs"), and they put their focus on presentation only. The activism styles and needs are different. TSs need full conformity to the other sex and see any issues in being accepted as more their own personal fault or that of their body. TGs tend to operate more as a community to themselves, don't really care about conforming ("just be yourself"), and see issues in being accepted as being society's fault. One tries to change themselves and the other tries to change everyone else.

And the above is just a rule of thumb. It's more a collection of profiles, and they must be taken with a grain of salt. The issue I see a lot in some is what I call the "potato problem." Everyone knows what an ideal potato looks like, and everyone knows what a completely rotten potato looks like. But it is harder to judge what is between. How many green spots are acceptable? What is an excessive number of "eyes?" Are tool marks allowed (like gouges from a shovel or pitchfork)? What about mild "blisters," bruises, or scrapes? So it is more of a subjective, intuitive art form than an exact science. I can usually tell the 2 apart, and others have said the same to me. I've run into transsexual folks (even those who reverted for whatever reason) who I could just feel the "girliness" oozing off of them, and I've had to refrain from wanting to stroke their hair. And then there were some that I could just read them right away, and while some of that pertains to their skill, I also notice the pattern of their voices (regardless of pitch), their demeanor, how they negotiate space (eg., "manspreading"), the topics they discuss ("I just got a new car," vs., "How is Emily, I haven't seen her, is she okay?"), the energy they give off (like defensive, belligerent, angry, territorial, detached), etc.