Hukot HaGoyim in this case is something done specifically by gentiles and therefore it would be prohibited (see S"A Yoreh Deah 178). So my question is, would jeans fall under this category?
So long as it isn't indecent, Rabbi Moshe Feinstein (Igros Moshe Y.D. 1:81) writes that it's not a problem. I've written it up here.
And for those who don't believe me, here's what it looks like in the original responsum:
Specifically regarding jeans it's even easier as we know where jeans come from -- gold prospectors who needed rugged clothing. If the origins of a practice are pagan, prurient, or make such zero sense that you'd obviously only be doing it to emulate the non-Jews, then it's a problem. None of that is the case here.
Rav Musafi says that in a place where people that dress modestly don't wear jeans, one shouldn't wear them. Therefor I would assume there is no issue of Hukot HaGoyim. http://www.doresh-tzion.co.il/QAShowAnswer.aspx?qaid=65856
He writes that it is only chukot ha-goyim if the non-Jews are the only ones who wear a particular type of clothing. If observant Jews also wear them, there is no such problem. As such it depends on the time and place.
He notes that if the pants stick to the body and outline its contours, then it is not modest for men, and all the more so, for women, no matter what the material is.