list all the ppa repositories added to my system

From How can I get a list of all repositories and PPAs from the command line into an install script?

Part of the answer looks to have what you are looking for:

#! /bin/sh 
# listppa Script to get all the PPA installed on a system ready to share for reininstall
for APT in `find /etc/apt/ -name \*.list`; do
    grep -o "^deb http://ppa.launchpad.net/[a-z0-9\-]\+/[a-z0-9\-]\+" $APT | while read ENTRY ; do
        USER=`echo $ENTRY | cut -d/ -f4`
        PPA=`echo $ENTRY | cut -d/ -f5`
        echo sudo apt-add-repository ppa:$USER/$PPA
    done
done

Save this as listppa.sh

listppa.sh > installppa.sh

This creates a script that you can backup somewhere, then run to add your PPAs on a fresh install by simply running:

installppa.sh

For those who just want to check the PPAs they have installed without actually doing anything with them automatically you can do:

$ apt-cache policy

In my system, here's a bit of what it shows:

% apt-cache policy
Package files:
 100 /var/lib/dpkg/status
     release a=now
 500 http: ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu/ precise/main Translation-en
 500 http: ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu/ precise/main i386 Packages
     release v=12.04,o=LP-PPA-ubuntu-toolchain-r-test,a=precise,n=precise,l=Toolchain test builds,c=main
     origin ppa.launchpad.net
 500 http: ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu/ precise/main amd64 Packages
     release v=12.04,o=LP-PPA-ubuntu-toolchain-r-test,a=precise,n=precise,l=Toolchain test builds,c=main
     origin ppa.launchpad.net
 500 http: ppa.launchpad.net/rael-gc/scudcloud/ubuntu/ precise/main Translation-en
 500 http: ppa.launchpad.net/rael-gc/scudcloud/ubuntu/ precise/main i386 Packages
     release v=12.04,o=LP-PPA-rael-gc-scudcloud,a=precise,n=precise,l=ScudCloud - Linux client for Slack,c=main
     origin ppa.launchpad.net
...

Quoted from here:

[apt-cache policy] retrieves priorities associated with each repository resource. From its output, you can infer a list of all available repositories and PPAs.

Source: http://ask.xmodulo.com/list-installed-repositories-ppas-ubuntu.html