If you use Xcode, a utility called ar is installed. You can run the following command:
ar -x path/to/filename.deb
If you don't use Xcode, you could also install ar using Homebrew and run the same command.
Alternatively, you could use Homebrew to install the dpkg utility which has many more options to work against .deb files.
dpkg -c path/to/filename.deb