To be a little bit more specific : the name of the admin directory is on the filesystem.
When you access a page of the admin directory, a script puts the current directory's path in the _PS_ADMIN_DIR_ constant.
If you forgot the name of the the admin directory you must have a look at the filesystem of your server.
Admin directories are automatically renamed to something like adminXXXX.
If you named it differently you can compare the default directory structure with your actual structure and find the proper directory.
You can also look for files that are only present in the admin directory. The "get-file-admin.php" file for example.
On linux, the following command run from the prestashop root directory will tell you the actual name of the admin directory :
find ./ -name get-file-admin.php
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