Instead of checking a list of well-known locations, you can ask pip to list the valid locations:
pip config -v list
Fun fact
On the same machine, with the same pip version, the valid locations can vary based on the actual Python version.
Environment: Win 7 x64, the HOME environment variable is set to D:\Home
Python 3.7.3:
> pip config -v list
For variant 'global', will try loading 'C:\ProgramData\pip\pip.ini'
For variant 'user', will try loading 'D:\Home\pip\pip.ini'
For variant 'user', will try loading 'C:\Users\foobar\AppData\Roaming\pip\pip.ini'
For variant 'site', will try loading 'C:\Python37\pip.ini'
Python 3.8.0:
> pip config -v list
For variant 'global', will try loading 'C:\ProgramData\pip\pip.ini'
For variant 'user', will try loading 'C:\Users\foobar\pip\pip.ini'
For variant 'user', will try loading 'C:\Users\foobar\AppData\Roaming\pip\pip.ini'
For variant 'site', will try loading 'C:\Python38\pip.ini'