Graduate students are a unique class of students that can be found in many major universities. While they are a varied group, there are certain characteristics by which one can identify them. Should you for some reason wish to identify these students, the following guide has been designed to help you.
Appearance: Of all the characteristics belonging to graduate students, appearance is probably the most varied. Graduate students range in age from anywhere from 22 on up. Most of them, however, are typically in their early to mid-20s. Their clothing often resembles their status -- somewhere between student and professional. Jeans worn with a nice top are typical, and reflect this dual status.
Habitat: When not engaging in "study breaks" (see Behavior), graduate students can typically be found in one of four locations. These are, in order of frequency, the building that houses their department, the library, in class, and their apartments. I say apartments, though some graduate students also rent or even own houses. The majority of grad students, however, rent one-bedroom apartments. Graduate student living spaces are typically better furnished than their undergrad counterparts, and generally contain a large number of books concerning their area of study.
Dietary Habits: The dietary habits of graduate students are wide and varied. They are typically healthier than those of undergraduates, but not necessarily. Often, they are a mix of quick, easy meals, and healthier, more involved meals. Breakfast is often the meal given the least amount of attention, as can be attested by the large quantities of granola bar wrappers that can be found in most students' cars. Lunch is typically a packed sandwich, leftovers for dinner, or a quick meal purchased on campus. Dinner ranges from frozen microwave meals, to proper meals with a meat and vegetables.
Behavior: Graduate behavior consists mainly of two activities- doing work (AKA "studying"), and avoiding work (AKA "study breaks"). The term "study break" covers a wide range of activities, from eating meals, to parties, to taking naps. Such behaviors are typically reserved for the weekend, and most graduate students spend their weekdays trying to get enough work done to justify taking a study break at some point on Saturday or Sunday.
Disclaimer: This is by no means a comprehensive guide, and the author takes no responsibility for its use. Please be aware that approaching a graduate student, especially one engaged in "studying" can be dangerous. Approaching a graduate student on a "study break" also carries it's own risks, including, but not limited to, being engaged in an in-depth conversation of an obscure field, witnessing a nervous break-down, and receiving a lecture on why one's chosen field of study is, in fact, "useful and important".