When I first started college, the Dean came in and said "Good Morning" to all of us. When we echoed back to him, he responded "Ah, you're Freshmen."
He explained. "When you walk in and say good morning, and they say good morning back, it's Freshmen. When they put their newspapers down and open their books, it's Sophomores. When they look up so they can see the instructor over the tops of the newspapers, it's juniors. When they put their feet up on the desks and keep reading, it's seniors."
"When you walk in and say good morning, and they write it down, it's graduate students."