There are multiple way on how to determine whether a specific package is already installed on your system. The first dpkg command will list all currenlty installed packages on your system:
# dpkg -l
Debian
Debian apt-get wheezy sources.list
Related sources.list repositories:
Wheezy,
Jessie,
Stretch,
Buster
Security Updates
# /etc/apt/sources.list :
deb http://security.debian.org/ wheezy/updates main contrib non-free
deb-src http://security.debian.org/ wheezy/updates main contrib non-free
Australia Mirror
# /etc/apt/sources.list :
deb http://ftp.au.debian.org/debian/ wheezy main contrib non-free
deb-src http://ftp.au.debian.org/debian/ wheezy main contrib non-free