54% of Icelanders don't deny the existence of elves, and 8% of believe that they exist, no questions asked. So when the aluminum company Alcoa wanted to build a smelting factory, the government required that an expert scoured the plant site to ensure there were no elves living there.
The process can take up to 6 months, but usually takes an hour. There is no way to train for it, one is just born with the ability to sense an elf about. It's thought that many that are born with the ability to see elves lose it at around age 8. It's said that elves go crazy if you try to build over their land, causing equipment failures, so keeping them happy is a priority.