RedOrbit.com has an interesting article about the steps scientists must make in order to claim that a species has gone extinct. They use the Baiji Dolphin as a prime example.

scientists must undergo endless searching to prove that any individual species has gone the way of the dodo.

"If there's one thing in my career I'd like to be proved wrong about, it's the baiji," said Sam Turvey of the Zoological Society of London, using another name for the Yangtze River dolphin.

Turvey interviewed Chinese fishermen for almost 3 months earlier this year, hoping to record sightings of the long-snouted dolphin, which has not been seen since 2002. Some colleagues in China are still looking.

In 2006, the baiji was almost declared extinct after an acoustic and visual survey of the river turned up nothing. Yet a blurry video forced experts to rate it "possibly extinct."