There's no perfect and easy way to determine and unfortunately they tend not to wear name tags that say transgender on them
Some of the more obvious signs include larger than normal hands, a large Adam's Apple and a deeper voice. Transgender from my experience is more prevalent in Thailand and Vietnam. Not sure if it's particularly common in Hong Kong as I don't recall seeing or hearing about many.
That being said, with the quality of surgeries nowadays, some of the more obvious signs may no longer be applicable. Sometimes it seems that if the girl looks TOO perfect or TOO beautiful, there may be a higher chance she's a transgender as they often look better than the real girls.
I think it's an underlying concern for ALL guys to accidentally hook up with one. Some deal with it better than others. Some may continue anyways thinking ahh what the hell while others will run screaming from the room with their pants around their ankles