I’ve long watched what people are watching on TV (nostalgia + dystopia), and at movies (safe sequels) to ponder emerging trends. I also keep an eye on what type of books are on the best-seller lists (cooking, monsters, travel, rebalancing) and even what slogans people have on their t-shirts (100% human), which can be especially useful to spot emerging weak signals.
However, over in Germany this idea has been made more rigorous. (Thanks to Aifric for spotting this btw).