What's the name of PPA? (for ppa-purge)
This command will list all the PPA repositories you have configured
grep ppa /etc/apt/sources.list /etc/apt/sources.list.d/*
Alternatively, you can check the repository of a given package
apt-cache policy [package-name]
I recommend using "Y PPA Manager" which has (among others) one nifty feature: it can list all packages from a ppa enabled on your computer.