ummm dont?
it really can only end two ways:
A) they are not TS, so you insult someone by insinuating that they used to be a man or some such thing or
B) they are TS and you insult them by saying "hey i just clocked you"
if... you REALLY need to know.. you may be able to get them to volunteer the information, by talking about it yourself or something and see if they are liek "oh yea im a TS too" or something... but.. eh i'd just leave it alone.