Once you paste any custom code and update your website, the code is added to your website's source code.
To check it, open your website, right-click on the page, and select Inspect:
Open the Elements tab, click CTRL+F (Windows) or CMD+F (macOS), and you'll see a search bar:
In the search field, you can either paste the whole code or just a keyword or phrase from your code (e.g., your Google Analytics ID):
This way, you should be able to find the integration in your website's source code 🕵️♂️