Edit in JSFiddle

$(function() {
    var availableTags = [
        "ActionScript",
        "AppleScript",
        "Asp",
        "BASIC",
        "C",
        "C++",
        "Clojure",
        "COBOL",
        "ColdFusion",
        "Erlang",
        "Fortran",
        "Groovy",
        "Haskell",
        "Java",
        "JavaScript",
        "Lisp",
        "Perl",
        "PHP",
        "Python",
        "Ruby",
        "Scala",
        "Scheme"
        ];

    $("#auto").autocomplete({
        source: availableTags,
        change: function (event, ui) {
            $(":submit").prop("disabled", !ui.item);
        }
    });
});

<form>
<input type="text" id="auto" />
    <input type="submit" disabled="disabled" />
</form>