Generally Thai girls are not very tall, but almost all katoeys are taller than average, with heights of 180cm not being unusual.
Look at the way she uses make up, the way she walks and moves her arms or hands, especially over-enthusiastic hair flicking... no real girl would need to act this way at every moment, like she is walking on a constant catwalk.
Overacts and Overreacts a lot:
Way more than necessary, often loudly in a high-pitched, exaggerated voice. It gets even worse in a conflict. Certainly, don't ever pick a fight with a ladyboy - you can't be sure of the outcome!
Too Large Breasts:
This again is not an absolute rule but too big and often provocative is a good indicator. They often cheerfully invite you to check 'how real they feel'... it's more a question of appreciating their financial investment.
This is always interesting - In Thailand, no matter how small the breast and how open-minded a girl is, even in gogo bars, girls always wear a bra! If she doesn't, especially when combined with points 1 and 4, you are probably talking to a katoey.
In the majority of the cases, voices - even trained voices - are still what gives many katoeys away. The pitch might be higher than men's voices but it somehow sounds unnatural.
This is a tricky one as Adam's Apples can be surgically removed, but if she has one, you can call her 'sir'.
The Dress Code:
Just like the overacting, the way a ladyboy dresses is usually a lot sexier than most girls. Add a generous amount of make up, feathers and glitter and you get a girl that has become so feminine and flamboyant that things start to get suspicious.
One of the most unclear indicators, supposedly men's elbows are more square than a girl's.
The ID card - no matter how feminine you are or if you use the ladies bathroom, even after a sex reassignment surgery, your ID will still show your real gender.