If one had to mark an exact "turning point" it would likely be WW1. Immense amounts of Old World wealth spent by previously top-level world powers such as Britain, France, and Russia to finance the war made it's way across the Atlantic, and the U.S. ended its historical tendency for isolationism and demonstrated how effectively it could ramp into war-mode. Went from being a weird outlier power to a global superpower within about half a decade, and the old superpowers lost much of their own power.

I'm sure there's earlier points where it would bear out statistically that the US was clearly richer/more powerful than continental Europe but World War II was when it became absolutely undeniable to everyone that it was the case. The Marshall Plan was basically all US money and it was the cornerstone to rebuilding the continent after the War.

I would also highlight the Suez Crisis in 1956 as a seminal moment where the US asserted that not only was it a preeminent world power but that it was THE Western world power and everyone else had to defer to it or face the consequences.