The trick is to simply add a "+" to the end of any Bitly URL. When you add the "+" the URL will redirect to Bitly instead of to whatever the original URL was. That will then show you the Bitly page on which the shortened URL is hosted and will show you what the original link was.
You can try this with my Bitly link of bit.ly/2ZZnuP2 which will take you to the registration page for my Practical Ed Tech Newsletter (https://practicaledtech.com/weekly-newsletter/). If you try the same link but add "+" to the end of it so it reads like this bit.ly/2ZZnuP2+ you will go to the Bitly page where I shortened my original URL.
I've created a couple of short video demonstrations of how this trick works. This is the Instagram video that I made about it. This is the YouTube video that I made about it.
I've created a couple of short video demonstrations of how this trick works. This is the Instagram video that I made about it. This is the YouTube video that I made about it.