Test for PhoneGap. (from: http://stackoverflow.com/questions/8068052/phonegap-detect-if-running-on-desktop-browser).

test-for-gap.js

/**
 * Determine whether the file loaded from PhoneGap or not
 */
function isPhoneGap() {
    return (cordova || PhoneGap || phonegap) 
    && /^file:\/{3}[^\/]/i.test(window.location.href) 
    && /ios|iphone|ipod|ipad|android/i.test(navigator.userAgent);
}

if ( isPhoneGap() ) {
    alert("Running on PhoneGap!");
} else {
    alert("Not running on PhoneGap!");
}