Does anyone know the most effective way to detect is my script added to the consumer site?

The core point of this problem is related to that a client might use some systems which will integrate a little bit later after javascript evaluation. (F.e. smth like Google Tag Manager or similar). In that situation, simply get of the page isn't going to work. I tried Puppeteer and Phantom techs which works with Chrome Headless feature but it's hard to deploy them. Maybe someone has some more neat idea?