How to find if file was successfully uploaded to webform
I wouldn't recommend doing this with javascript, you should use a PHP function to inform the client-side of success.

However, if you insist on doing it,  you could (in theory) just create a new AJAX call to the location wehre you expect the new file to be.   If it errors,  it means it wasn't uploaded.

$.ajax({
    url:'http://www.example.com/yourfile.jpg'
    error: function()
    {
        //file not there.
    },
    success: function()
    {
        //file is there
    }
});