I get an idea because I get emails from Old Navy and DressBarn, and Kohl's. I see a lot of the same types of clothing in the email pics, and I figure those are currently "in."
Otherwise, I have no clue, nor do I really care. I have some "vintage" clothes I wear (can I call them vintage if they've been sitting in my closet for 20 years? Not sure they were "in" when I first put them on 20 years ago) and I have newer stuff I wear. I've actually been complimented on my clothing several times by someone almost half my age, so I guess I'm doing ok.
I primarily go for comfort and somewhat flattering. If it's "in," even better