A super simple user script with an unobtrusive way of being clear that it's running.

red-border.user.js

// ==UserScript==
// @name        Red Border
// @description A super simple user script with an unobtrusive way of being clear that it's running.
// @namespace   test
// @include     http*
// @version     1
// @grant       none
// ==/UserScript==

document.body.style.border = '3px dashed red';