How to know whether there's an Boolean arg passed in? function test(val:Boolean = ?)

using:

function test(val:String = null):void {

}

//would easily know whether there's a string passed in:

test(); // No string passed in
test("1"); // Found string passed in

using:

function test(val:Boolean = null):void {
  
}

is not working. in this case, I want the function know whether there’s a boolean arg passed in.