Best way to validate from fields before ajax submission

you can use client side validations:

using Query:
http://www.jquerybyexample.net/2013/01/client-side-validation-jquery.html

using JavaScript:
http://jquerytools.org/demos/validator/server-side.html

Most people use this

http://bassistance.de/jquery-plugins/jquery-plugin-validation/

is it the best approach to create a plain java script method to do the validation and then check in the another function its return value and then call ajax to submit the form?

function validate(){
xxx
xxx
xxxx

return true/false
}

function SubmitFrm() {
 var chkfrmValidation = validate();
//if the return value true go for ajax
       {
            $.ajax({
                  type : 'post',
                  url : 'testsubmit',
                  data : $('#registerform').serialize(),
                  async : false,
                  dataType : "text",
                  beforeSend : function() {
                        $.blockUI({ message: '<h1><img src="../img/loading.gif" /> Processing...</h1>'});
                  },
                  complete : function() {
                        $.unblockUI();
                  },
                  success : function(data) {
                  $.unblockUI();
                        alert(data);
                        
                  },
                  error : function(data) {
                        // check status && error
                  }

            });
      }

What if we call the validate function in   beforeSend : function() {} ?

Why even start the ajaxing if not valid?

Why even start the ajaxing if not valid?