While the most common way of testing one’s sobriety is blowing into a breathalyzer machine, it’s far from the only way. At roadside checkpoints, one thing police often look at is called horizontal gaze nystagmus (HGN).

HGN can be caused by several factors, but alcohol consumption is prime among them. When drunk, our eyes don’t want to move smoothly, and this can be easily observed by a sober policeman with a penlight.

It can also be observed by the camera of an iPhone 4 or iPhone 4S, and HGN is exactly what’s measured by the BreathalEyes app. As Gizmag points out, the app comes chock full of disclaimers, so even if BreathalEyes says you’re sober, you’ll still want a second opinion or third opinion before climbing behind the wheel.

Disclaimers aside, Wired says that the BreathalEyes developers claim accuracy within 0.02 percent of the indicated blood alcohol content (BAC) when tested alongside a breathalyzer.

The app has a range from .04 percent to .17 percent BAC, which is plenty broad considering that most states now count .08 percent BAC as legally drunk, with .05 percent BAC constituting “ability impaired” in some locations.

If there’s a drawback to the 99-cent app, it’s this: using it requires two people, one of whom must be sober enough to hold an iPhone steady and count to ten. The test subject faces the camera while his eye is framed, then looks left (left eye) or right (right eye) for 10 seconds.

If you manage that, the app will spit out an approximate BAC, but we’re not sure we’d trust a 99-cent app when thousands of dollars worth of legal fees (or worse) are on the line. When in doubt, call a cab.