Earth's counter-attack to alien invasion is McDonalds.
Numerous movies depicting subterfuge alien occupation (Alien Nation, 3rd Wave etc) show salt as lethal to them. So if you suspect someone is an alien buy him / her some salted McDonald's French Fries.
If they decline, they're hiding the fact that they're aliens - kill them.
If the accept and don't die, they've mutated - kill them.
If they die, well there's your proof.