Just use the event onload to the iframe.
Take an example:
    window.addEventListener('load', function(){
       var iframe = document.querySelector('iframe'),
           carregar = document.querySelector('#carregar');
        carregar.addEventListener('click', function()
        {
            // mudamos o src do iframe
            iframe.src = 'include.php?' + new Date()
        });
        iframe.addEventListener('load', function(){
            alert('carregou o conteúdo do iframe')
        })
    })
HTML:
 <iframe></iframe>
 <a href="#" id="carregar">Carregar</a>
I made the example by changing the src of iframe through the click event #carregar so that it can become more noticeable at the time of testing.