How do you know your teacher is gay?

The only way you can conclusively know if your teacher is gay is if he or she tells you about their sexual orientation. Despite the many stereotypes and generalisations people use, these are inaccurate and do not necessarily indicate a person's sexual orientation.

You should also think about why you even want to know. Does the teacher's sexual orientation have any impact on you or are you just being nosy? There is usually no need to ask such questions about the teacher.