[kingaillas]

The evidence is likely simple deduction, as in asking "when was the last time this area was bombed" combined with the history of the airport (built for the military in 1943, later converted to civilian use) and also noting other unexploded bombs have been unearthed in the area.

For it NOT to be a WW2 bomb would mean somebody sneaking in another bomb and paving it under the runway without being noticed.

[ImJamal]

There is a perfectly plausible alternative. A bomb fell off a plane after the war. This happens from time to time and has even happened with a nuclear bomb!

[crote]

Yes, and it results in a huge amount of paperwork. You can't just randomly show up at your destination missing a bomb - especially during peacetime. They'll be combing over every part of your flight path to find it.

It could've also been intentionally buried there by bored soldiers, or placed there by airport maintenance people as a prank, or ended up there due to a freak teleporter accident. Maybe it was even put there by Godzilla. Maybe the Infinite Improbability Drive spontaneously materialized it, together with a bowl of petunias.

If you find a bomb in an area which is known to have been bombed, without any evidence to the contrary it is pretty safe to assume it's there due to the bombing.