Nobody times it exactly and nobody knows the exact moment "just before the crash".

The smart traders usually take some money off the table in stages - they set themselves targets and obey those targets and don't cry if the price goes up after they sell.

I think it was the Rockerfellers who said that they got rich by "selling too early". What they meant was they were not too greedy and understood that it is impossible to predict the exact highs and lows, so you look for reasonable profits and book them.