Take your comment and change any gendered words from male to female and ask yourself “Would I say this to or about a man in the same situation?”

For example:

• “This office is a lot prettier than when I was here” [to a male graduate student, while thumbing the female grad student hard at work] doesn’t change, and your answer would likely be no.
• “Little girl, you will have to work a lot harder to keep up with the men!” [to a student in the first tutorial of first year physics] becomes “Little boy, you will have to work a lot harder to keep up with the women!”, and your answer would likely be no.
• “Be careful, you wouldn’t want to be mistaken for a secretary.” [to a female postdoc as she consults a dictionary to settle a disagreement] doesn’t change and your answer would likely be no.
• “You should be at home, raising children. That is what women are good at.” [to a female professor in her own office] becomes “You should be at home, raising children. That is what men are good at.”, and your answer would likely be no.