How can you tell if someone is a transsexual?

"IT"'s never anything. The only person who ever should be asking about "telling" if a person is transsexual, is the person themselves as in "Am I transsexual?" It's none of your business what is another person's chromosomal design, nor is their gender configuration any of your business. I don't ask if your "___" bends to the left or your "______" is hairy, don't ask me about mine.

And with modern techniques, maleTofemale (m2f) reconstructive surgery is so good, without going further than most women care for a gynecologist to go or using xrays or mri's, the closest a Doctor can come to actual birth gender is by assuming the patient had a hysterectomy.

Most people haven't learned yet that human's don't come in just 2 sexes - human sexuality is much more complex than that: it really is a spectrum from female to male. You can be born with male parts externally, chromosomes of XY and yet have some or all female parts internally. You can be born with XX as your chromosomes, yet you have testes internally. You can be born with parts that are partially both and neither. The more complex the design of a thing, the more likely are variants in design and form.
When they tell you.