Many long hours in the lab, observatory or one's own head can leave many dedicated physicist unable to make the basic distinction between the sexes.
Fortunately, there is a handy reference for identifying the sexes that can prevent many an unfortunate encounter. This is similar to the process for confirming a hypothesis, and contains many null hypothesis, thus it is totally scientific and can be used without worrying about pesky emotions.
The first step is to check the shoes. Shoes are almost always a great way of identifying men and women from one another. Women have a tendency to wear narrow, shiny and colourful shoes, men wide, matt and dull ones. Mostly, men's shoes are flat, whereas quite often, women's shoes will be heeled, with thin heels.
Shoes is just the first step though. In isolation they cannot be used to confirm or refute the gender of the person. Partly because some men wear high heels, partly because some women wear dull, matt shoes and partly because sometimes staring at people's shoes is rude. A false positive here will generally only result in a woman being classed as a man, which is possibly not a bad thing.
Hair can be a good indication. If the person's hair touches their shoulders, there is a fair chance that they are a woman. Most physicists will only know bald men, but as with swans, not all men are bald. Therefore, the hair tenet is a tenuous one; one that MUST be evaluated along side all other tenets. Quite often, only hair is evaluated, and this has led to many an awkward moment with somebody who rides a very large motorcycle and has the name Hank.
Habits are a useful method for determining the gender of a person. If they put containers back into the fridge containing just one sip of juice, half a millilitre of milk or a combination of the two in the ketchup bottle, they are a man. If they complain at this being done, they are a woman. This is a certain method for identifying the sexes, but it is hard to garner this information without a restraining order being imposed. Other male habits are wet towels left well away from bathrooms, leaving toilet seats up and being able to reverse a car. Women's habits include talking, listening and remembering every little thing you ever did wrong.
Clothes, make up and shape are quite often good identifiers. Trousers can be worn by either sex, but in general, skirts are worn by women, the same is true of make up (this is facial paint). This is not always the case. However, men who wear skirts and make up are either transvestites or Scottish. Both will show you a good time, but in very different ways. They can be distinguished by the colour of the make up, if it is blue, they are Scottish. Women tend to have narrower waists than hips, whereas in men it is the other way round. Taking shape, clothes, make up, hair and shoes together can give a good indication.
The best way of identifying a woman though is yet to be disproved, and has been honoured with the title of "the Law of Sexual Identification". If upon announcing you are a physicist they say "Wow" they are a man. If they tut and turn away to look for somebody else, they are a woman.