Find out if something has changed in the form using jQuery

You do not need jQuery.

document.getElementById('campo').addEventListener('input', function () {
    /* faz alguma coisa */
});

You can use change instead of input if you want your code to run only when the user exits the field.

If you want to do this for everyone, you can do

var toArray = Array.prototype.slice.call;
function doSomething() {
    /* faz alguma coisa */
}
toArray(document.querySelectorAll('input[type="text"]')).forEach(function (input) {
    input.addEventListener('input', doSomething);
});

You can also use the jQuery change function, it looks like this:

$(':input').change(function(){  
  //Ação desejada.
}); 

In this small example I checked to select all the INPUT fields, but you can select the desired fields from the jQuery selector.

In Desired Action, you place the code to be executed when a user modifies the selected field, such as giving feedback that the field has been filled in.