Epa aqui te va un codiguillo que encontre por ahi:
<html>
<head>
<!-- Copiar dentro del tag HEAD -->
<script language="JavaScript">
<!--
function detectarflash(){
//********cambiar los siguientes datos*************
flashpage = "intro.html"
upgradepage = "plugin-flash.html"
nonflashpage = "sin-flash.html"
cantdetectpage = "no-se-pudo-detectar-plugin.html"
//**************************************************
noautoinstall = ""
if(navigator.appName == "Microsoft Internet Explorer" &&
(navigator.appVersion.indexOf("Mac") != -1 //
navigator.appVersion.indexOf("3.1") != -1)){
noautoinstall = "true";
}
if (navigator.appName == "Microsoft Internet Explorer" &&
noautoinstall != "true"){
window.location=flashpage;
}
else if(navigator.plugins){
if(navigator.plugins["Shockwave Flash"]){
window.location=flashpage;
}
else if(navigator.plugins["Shockwave Flash 2.0"]){
window.location=upgradepage;
}
else{
window.location=nonflashpage;
}
}
else {
window.location=cantdetectpage;
}
}
// -->
</script>
</head>
<!-- Copiar dentro del tag BODY -->
<body onLoad="detectarflash()">
</body>
</html>