Also simple javascript check:

var c = document.getElementsByTagName('select'); //collection
for (var i=0, l = c.length; i<l; i++) {
    alert(typeof c[i].attributes['multiple'] == 'undefined' ? 'single':'multiple');
}

And jQuery equivalent:

$('select').each(function(){
  alert( typeof this.attributes['multiple'] == 'undefined' ? 'single':'multiple' );
});

All the options except ":select[multiple]" (shd be "select[multiple]") you tried should work.

JSFiddle: http://jsfiddle.net/VAXF6/2/

However you are missing a closing paran for your if statement.

Change your code to:

if($("select[multiple]").length){
           alert("worked");
}

or

if($("select").attr("multiple")){
           alert("worked");
}

or

if($("select").attr("multiple") != 'undefined'){
           alert("worked");
}

Another alternative:

if($("select").is("[multiple]")){
           alert("worked");
}

It seems you need to only alert if multiple was set with a value, not just if it exists as an attribute:

if($("select[multiple='multiple']").length){
    alert("worked"); 
}

You could try using one of the following boolean indicators that has not been proposed:
let selects = document.getElementsByName('multi_values[]'); // returns collection
Method 1 : hasAttribute()

for (let i = 0; i < selects.length; i++) {
    const select = selects[i];
    if (select.hasAttribute('multiple')) {
        alert('Multiple select found');
        break;
    }
}

Method 2 : bang bang Element.attributes

for (let i = 0; i < selects.length; i++) {
    const select = selects[i];
    if (!!select.attributes['multiple']) {
        alert('Multiple select found');
        break;
    }
}