Google "call mvc action from jquery" and you'll find lots of examples. Just make sure you have type:"GET" rather than type:"POST"
http://stackoverflow.com/questions/4120212/mvc-ajax-json-post-to-controller-action-method[
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You'll need something to trigger your call, so either use a click event of a button or link, or use the blur event of the textbox if you want the call to be made when the user tabs away. google "jquery blur event input box" if you don't know how to attach code to element events.
To pre-empt your next question, no I won't write the code for you.