Check the browser userAgent and determine if it is IE8+ or IE7- (counting compatibility mode as the +, not the -)

ieBrowserVersion.js

// usually code that works in IE8/9/10 will also work in IE8/9/10 in compatibility mode, but IE reports itself as IE7
// or some such when running in compatibility mode... so let's pretend that IE9 in IE7 compatibility mode is IE9
function checkVersion() {
    var msg = "Not IE";
    var ver = -1; // assume failure
    var ua = navigator.userAgent;
    if (navigator.appName == 'Microsoft Internet Explorer') {
        var re  = new RegExp("MSIE ([0-9]{1,}[\.0-9]{0,})");
        if (re.exec(ua) != null)
            ver = parseFloat( RegExp.$1 );
    }
    if (ver > -1) {
        // the version is usually valid
        if (ver >= 8.0)
            msg = "IE8+";
        // unless we're in Compatibility Mode
        else if (ua.indexOf('Trident') !== -1)
            msg = "IE8+CM";
        else
            msg = "IE7-";
    }
    alert( msg );
}