Mitchell: You don't have beards so you're not from the evil twin universe.
This is a reference to the "Mirror, Mirror" episode from the original Star Trek series. Kirk, McCoy, Scotty and Uhura enter an alternate universe where the Federation is evil and warlike. The "evil" Spock has a goatee. The "evil selves have beards" continued on into other Trek franchies mirror-universe episodes, and even shows like South Park.