You can access them using
var l = $("#yourlabelid");
The issue is how you get the label id. If your js is on the same master\content page as your controls you can use
var l = $("#<%= YourControl.ClientID %>");
If it's not, then create a variable on your master\content page that contains the id
var myLabelId = '<%= YourControl.ClientID %>';
You can then use mylabelId in your js
var l = $("#" + mylabelId);
The only thing you need to watch for is that you don't try and use the variable before it is defined.