They did it by laboriously counting and mapping clusters of lead atoms in the zircon using what's called atom-probe tomography.
"It's astonishing to be able to do this," said Valley, "literally counting atoms with an atomic probe."
Valley and his colleagues found that the lead was indeed getting kicked around, but it wasn't going far enough to throw off the age of the zircon.
"I think it settles it, as far as it's possible," said Valley.
It also points the way for techniques that can be used to better study zircons from beyond Earth, said Valley. There are, for instance, zircons from meteorites that are older than the Earth -- up to perhaps 4.6 billion years old.
And Moon rocks have zircons that are dated to 4.44 billion years old (this fits with the idea that the Moon solidified more quickly because it is smaller than Earth).
In fact the Valley's research is funded, in part, by NASA's Astrobiology program in anticipation of the day when a sample-return mission to Mars brings back aspirin-sized rock samples for analysis.