Really no way to check if a FL is a real woman or trans...
Now a days medical operation is so advance, not only do they cut and add items to a man body to make if look like a woman, they even have the capability to inject female hormones to force the man body to adopt the woman body of hair grown, muscle realignment etc
Hormone replacement therapy
The only way cannot siam is to ask the suspect FL for his/her ID lor i think you cannot suka suka tell the government to change your gender on their ID record (I may be wrong in that)
EDIT : I notice some countries allow a trans to change their Gender on ID
Thailand you just look at the Title on the ID Card (Mr, Miss/Ms), from what I know there was a bill presented in 2008 to allow the change of title on ID card but it was not approved