Enter the Zombie Survival Guide Scanner iPhone App.
All you do is point your iPhone camera at the suspected infected's face, line them up in the scanning sensor and hit the button. In seconds you'll be told if they are infected and how far it's gone -- and you'll see that info represented as a chewy, face-shredded representation of your once-friend, soon-to-be zombie nemesis. Then you can save it, and more importantly, share it, in case you get eaten before you can warn your comrades of the menace in your midst. See the video below for a demonstration:
Okay, in truth you shouldn't rely on this in case of an actual zombie outbreak (rely only on your wits, a machete and perhaps a shotgun in that case), since it's a gimmick to promote Max Brooks's series of zombie books -- The Zombie Survival Guide, World War Z and the graphic novel The Zombie Survival Guide: Recorded Attacks. But considering those are some of the most important works in the zombie canon (if you haven't read them, stop what you're doing and get to it), that it's a free app and that you can use it to make grotesque zombifications of your friends and family, even while on the go, it's still a must-have app. Get more information and a link to the app on the iTunes store from the Zombie Survival Guide Scanner website.