The only reliable way to find out someone's gender is to ask them what gender they currently think they are. There are more than two genders and an individual's gender can change with time.
A better strategy all-round would to be ask yourself whether you need to know someone's gender. I can think of very few legitimate reasons to know and record a user's gender and many of them can be dealt with by simply asking them what they'd like to be referred as, perhaps in more than one scenario.
A better strategy all-round would to be ask yourself whether you need to know someone's gender. I can think of very few legitimate reasons to know and record a user's gender and many of them can be dealt with by simply asking them what they'd like to be referred as, perhaps in more than one scenario.