Check if same origin policy applies

Question

Is there a "safe" way to check if the same origin policy applies to an URL before actually trying to use ajax methods? Here is what I have:

function testSameOrigin(url) {

    var loc = window.location,
        a = document.createElement('a');

    a.href = url;

    return a.hostname == loc.hostname &&
           a.port == loc.port &&
           a.protocol == loc.protocol;
}

This sort of works, but it’s kind of a manual guess based on the wikipedia article. Is there a better way of pre-checking cross domain allowance? jQuery is OK to use.