I think if I actually got confused to the point of believing it was a chick, I'd just go through with it anyway. Coming across the gear between the legs would be the deal breaker, so post-op and my senses can't tell the difference, game on.
Also, whenever I see a strikingly good looking chick, like the kind that make you do a double take, the first thing that pops into my head is always "if that was a dude, I'd STILL fuck it she's that fine!"
The Asian trannies are the toughest ones to distinguish between IMO.