I'd go with that.
6' is unusual for a woman, but not impossible, and increasingly common with better nutrition - I've known five women I can think of straight off the bat over 6', including my own grandmother, who was born in the early 1900s - a time when it was unusual for even men to be 6'.
So it's an outlier not an impossibility. And as stated, she's not even 5'10", and she's Malaysian, not Thai it seems, who are taller than Thais generally and of a more mixed genetic heritage, including western.
Things I'd look out for? Thick brow ridge, angular jaw bone, Adam's apple, broad shoulders, unusually large hands, lack of prominent hip bones, lack of foot arch. None of these in isolation point to an ex-male - many Thais don't have prominent hip bones for instance, and there's plenty of Sloane types with angular, horsey faces.
Also, feet are usually in sync with height and weight, so a tall woman's going to generally have bigger feet to support the higher weight that comes with being taller.
There's also plenty of women who LOVE anal sex. A quick search on AW of females in the South West returns 1,404 profiles. Add a filter on "Likes 'A'-Levels" and you get 200 - so one in 7, or 15%.
Add in At discretion, and you're nearly up to 250, or 1 in 6 who offer anal sex.
I doubt one in six women are trans-sexuals - again, AW says 23 out of 1,404, so 1.6% or 1 in 60.
All of the above are factors, but it's only when you start to see more than a couple of these in combination, that you up the odds. One or two in isolation are not proof or a concern.
