An admin panel of website is like a management system used by its admins or editors to manage the website  like adding pages, images, changing website layout etc.

To access the admin panel you need these two things:
1. Credentials (username and password)
2. URL of the login page, for example

Now the most important question

Well there is no 100% working method for finding admin panels but all we can do is to try.
Lets start with exploiting human stupidity, I mean some stupid admins use URLs that are too common to guess like,, etc.
Checking these links manually is too time consuming so you can use this online admin finder which checks for 2500+ possible URLs.