Edit in JSFiddle

function initializePhoneGap( success, failure ) {
    var timer = window.setInterval( function () {
        if ( window.device ) {
            window.clearInterval( timer );
            success();
        };
    }, 100 );
    window.setTimeout( function () { //failsafe
        if ( !window.device ) { //phonegap failed
            window.clearInterval( timer );
            failure();
        };
    }, 5000 ); //5 seconds
};

window.onload = function () {
    initializePhoneGap( function success() {
        //start app
        document.getElementById( 'result' ).textContent = 'phonegap: success';
    }, function failure() {
        //phonegap failed 
        document.getElementById( 'result' ).textContent = 'phonegap: failure';
    } );
      
};

should fail on desktop after 5 seconds
<div id="result">phonegap:</div>













































































































































































































<script>
    initializeHeader( 
        'Phonegap - onDeviceReady() wasn\'t fired',
        'http://stackoverflow.com/q/15606870/918414'
    );
</script>