When in doubt, log out of all Google stuff and visit Youtube. It will ONLY show you things that are popular/trending if you do this because Google won't have anything else to go off of. This is one very fast and easy way to see what's trendy on Youtube right now. For ultimate safety you can also make a brand new Google account on a "test device" like an old cell phone and never click or mouse over any videos. Just scan over the page and take note what's there.
To predict trends, look at Google search patterns and hop on. Upcoming events/films/toys/ect often are safe bets for trends.