@Drewid's answer didn't work in my Firefox 25 if the flash plugin is just disabled but installed.
@invertedSpear's comment in that answer worked in firefox but not in any IE version.
So combined both their code and got this. Tested in Google Chrome 31, Firefox 25, IE 8-10. Thanks Drewid and invertedSpear :)
var hasFlash = false;
try {
var fo = new ActiveXObject('ShockwaveFlash.ShockwaveFlash');
if (fo) {
hasFlash = true;
}
} catch (e) {
if (navigator.mimeTypes
&& navigator.mimeTypes['application/x-shockwave-flash'] != undefined
&& navigator.mimeTypes['application/x-shockwave-flash'].enabledPlugin) {
hasFlash = true;
}
}