I'm not sure how you can tell. It's easy to form opinions like this about undergraduates that you're teaching, but in practice they're based on virtually nothing. You don't know these people, you just interact with them in a very restricted way for a few hours a week. (And you spend hardly any time at all with them on a one-to-one basis.) At least, that's how I fell when I'm teaching.
I don't think first-years are a fair assessment of any institution. Many of the original 90% partiers will either a) shape up, b) drop out of college or c) some other choice. Good luck -- my friend is also a grad student teaching first-year calculus for "business, social sciences and humanities" .. ie kids that aren't interested in math. In his words, "it's painful".