It's simple, set a specific value for that label (XXXXXXX for example) and run it, open html source of output (in browser) and look for XXXXXXX, you will see something like this <span id="mylabel">XXXXXX</span>
it's what you want, the ID of <span>
(I think it's usually same as Label name in asp code) now you can get its value by innerHTML or another method in JQuery