you can use keyup() event:

$('#someInput').keyup(function() {
    // Your code here
});

there is also an input event:

$('#someInput').on('input', function() { 
    // Your code here
});

as frnhr pointed out, it's better to use input event because: "input will handle any change - e.g. if you paste something in the field, input will trigger but keyup wont."