Can I use JavaScript to check whether JQuery is (already) downloaded on the target web browser (user) or not? For Example:

If (JQuery-from-Microsoft-CDN-downloaded)
    Then use http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.4.4.js
Else if (JQuery-from-Google-APIs- downloaded)
    Then use http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js
Else if (JQuery-from-code.jquery.com- downloaded)
    Then use http://code.jquery.com/jquery-1.4.4.min.js
Else use jQuery from my own website.

Means that using the ability of javascript to check whether one of them is downloaded on the target User (Web Browser), if not then use jQuery from my own website otherwise if true then use that version of JQuery that is downloaded on the target User.