Ask HN: How do you identify trends? Where do you look?

I'm no soothsayer, but I've been curious about staying up to date with trends among certain communities, such as: streetwear fashion, startups, college students, and technology (a random assortment of interests, I know).

I mostly use Reddit, Instagram, and HN to qualitatively look out for trends, but I am struggling with how to apply a more quantitative methodology to validate my own assumptions. I've used Google Trends, but am not sure what the best keywords are, and also don't know of better data sources than Google Trends to look. I've never been an analyst or marketer, so I'm inexperienced in this regard. I greatly appreciate any advice or suggestions.