Javascript checking if page is valid

Question

On my submit button, what I'd like to do is OnClick show a "Please wait" panel and hide the button, UNLESS the validators say something's invalid - then I need the buttons still showing obviously. Otherwise I have a validation summary showing erros and no way to submit again.

Most articles I find about doing this want to use Page_ClientValidate() function to tell the page to validate itself, but this comes back undefined for me, as does Page_IsValid variable. Here is the function I'm trying to use - what am I missing?:

function PleaseWaitShow() {
    try {
        alert("PleaseWaitShow()");

        var isPageValid = true;

        // Do nothing if client validation is not active
        if (typeof(Page_Validators) == "undefined") {
            if (typeof(Page_ClientValidate) == 'function') {
                isPageValid = Page_ClientValidate();
                alert("Page_ClientValidate returned: " + isPageValid);
                alert("Page_IsValid=" + Page_IsValid);
            } else {
                alert("Page_ClientValidate function undefined");
            }
        } else {
            alert("Page_Validators undefined");
        }

        if(isPageValid) {
            // Hide submit buttons
           document.getElementById('pnlSubmitButton').style.visibility = 'hidden';
           document.getElementById('pnlSubmitButton').style.display = 'none';

           // Show please wait panel
           document.getElementById('pnlPleaseWait').style.visibility = 'visible';
           document.getElementById('pnlPleaseWait').style.display = 'block';
       } else {
           alert("page not valid - don't show please wait");
       }
   } catch(er) {
       alert("ERROR in PleaseWaitShow(): " + er);
   }
}