gistfile1.js

function getClass(obj, useNative) {
  return Object.prototype.toString.call(obj, useNative)
    .match(/^\[object\s(.*)\]$/)[1];
}
 
function Foo(){}; // new foo == "[object Foo]"
 
var Bar = function(){}; // new Bar == "[object Object]"
Bar.name = "Foo"; // toStringX doesn't rely on function.name so this won't matter
 
var tests = ["new Foo", "new Bar", "123", '"a string"', "false", "new Function"],
results = [];
 
for (var i=0; i<tests.length; i++) {
    results.push("getClass(" + tests[i] + ") == " + getClass(eval( tests[i] )));
}
 
results.push("getClass(new Foo, true) == " + getClass(new Foo, true))
 
alert(results.join("\n\n"))