Even today when the internet seems to shorten the time it takes a fad to spread worldwide, there still exists a delay.
Overlay a world map with the timeline of fads’ spread. Show how and when it arrived and became a trend in each country.
Most trend analysis is being published by someone who has some other agenda, and that skews the results.
Start with particular interest area(s) like fashion, online games, technology, etc. The main important point is that it is independent. It’s vetted and curated by experts in the field who are also vetted for no hidden agendas. This will give you a clear view of fads and trends.