Check all Checkboxes in Page via Developer Tools

(function() {
    var aa= document.getElementsByTagName("input");
    for (var i =0; i < aa.length; i++){
        if (aa[i].type == 'checkbox')
            aa[i].checked = true;
    }
})()

With up to date browsers can use document.querySelectorAll

(function() {
    var aa = document.querySelectorAll("input[type=checkbox]");
    for (var i = 0; i < aa.length; i++){
        aa[i].checked = true;
    }
})()