A new paper says they can detect sexism in a smile. 

A man’s true attitude towards the female sex can be detected according to how he smiles and chats to her, according to Jin Goh and Judith Hall of Northeastern University writing in Sex Roles

Isn't that too subjective, you may ask? Certainly a woman who has used the term 'sexist douchebag' more than once probably sees sexism in a lot of places that mentally stable women do not, but like with tests that set out to show how racist you are, saying that all men are sexist but some are just more overt about it doesn't seem to work outside the social sciences. That is why benevolent sexism was invented - if a man tries too hard to help women, he can be just as sexist as a man who penalizes women and it is all up to a woman deciding on a case by case basis. 

Is he sexist? He sure is smiling a lot. Credit: University at Albany

Goh and Hall wanted to prove that men’s word choice, attitudes and smiles show the type of sexism they show when interacting with women they have just met. To make their case, they categorize nonverbal and verbal expressions of benevolent and hostile sexism during mixed-gender interaction, and how these two types of sexist beliefs are expressed differently.

The study was small, and done with college students, like almost all social science papers are, in this case 27 papers from, unsurprisingly, their school. The students were filmed while they played a trivia game together and then chatted afterwards. Observers then scrutinized their interaction by reporting their impressions and counting certain nonverbal cues such as smiles. Word count software was also used to further analyze the content.

The more hostile sexist men were perceived as less approachable and less friendly in their speech. Men with more hostile sexism also smiled less during the interaction. In turn, those who displayed benevolent sexism were rated to be more approachable, warmer, friendlier and more likely to smile. They also used more positive emotional words and were overall more patient while waiting for a woman to answer trivia questions.

In other words, there is a very small 'sweet spot' for behavior. If you do not show favoritism toward a woman, because you do not want to date her or be friends, that is hostile sexism. If you were friendly, you were benevolently sexist. 

“While many people are sensitive to sexist verbal offenses, they may not readily associate sexism with warmth and friendliness,” argues Goh. “Unless sexism is understood as having both hostile and benevolent properties, the insidious nature of benevolent sexism will continue to be one of the driving forces behind gender inequality in our society.”

“Benevolent sexism is like a wolf in sheep’s clothing that perpetuates support for gender inequality among women at an interpersonal level,” elaborates Hall. “These supposed gestures of good faith may entice women to accept the status quo in society because sexism literally looks welcoming, appealing, and harmless.”

Citation: Goh, J. X.&Hall, J.A. (2015). Nonverbal and Verbal Expressions of Men’s Sexism in Mixed-Gender Interactions, Sex Roles. DOI 10.1007/s11199-015-0451-7