Hello
I looked at BTC, ETH, XRP and BCH on coinmarketcap. At around 7:30 UTC their prices dropped dramatically.
How could one have known in advance or at the same time or at least as soon as possible?
You couldn't have known in advance. It was triggered by comments from the Korean finance minister hitting the news sites. The only people who would have known in advance were those working for the Korean finance ministry.
That said, Bitcoin rose too fast and many people were looking for an excuse to take profits.