How did they find out? Probably through contact with people who work with the Americans — in other words, this is likely another indication of the fact that there is no firewall within Muslim communities between jihadists and peaceful Muslims. Jihadists have never been expelled from Islamic communities, but rather move freely within them. And so things like this are virtually impossible to prevent.
An update on this story from AFP, with thanks to the Constantinopolitan Irredentist:
Islamabad – A suicide attack at an Afghan air base where US vice-president Dick Cheney was staying shows that the Taliban and al-Qaeda have penetrated local intelligence agencies, analysts and officials said.
The blast early on Tuesday at Bagram air base near Kabul also highlights the increasing sophistication of the extremist outfits as they prepare for a feared spring offensive against Western troops, they said.
[…]
“This shows how much the militants have penetrated the intelligence of the Afghan security forces. It is a most shocking attack,” retired Pakistani general turned analyst Talat Masood told AFP.
Visit unannounced
Cheney’s visits to Pakistan and Afghanistan were unannounced and shrouded in even tighter secrecy than when US President George W Bush travelled to the two countries in March 2006.
[…]
A senior Pakistani counter-terrorism official said the “sophisticated” attack “indicates the militants’ preparedness and the quality of their intelligence collection in the run-up to the so-called spring offensive”.
He added: “They must have had information (a) few days before that the US vice-president would be in town and stay at Bagram. This is not something you can plan with 12 hours notice.”