It would be foolish to assume that every TS is easy to detect, but my personal experience is that on a number of occasions I have been able to discern relatively easily a TS in public (bus, train etc). I find hands, feet, throat and voice are the common give always. I have punted Thais on a couple of occasions and both were, I am absolutely sure, women but I no longer punt Thais because there is currently such poor value in this sector in the U.K. as Bill Goldberg (Thai officianado) will confirm. This said, there certainly were recent Thai agencies in London representing Thai TS prostitutes as genetic women to unsuspecting heterosexual men - there was a thread on this about a year ago.
Which is all a roundabout way of saying - there is a risk, it is manageable, but are you sure you want to bother?