Iptables in Linux is implemented as a number of kernel modules. Normally it is configured by some init script - but you can add and remove rules on the fly from the command line. So basically, there are (at least) thee relevant questions:
- Are the kernel modules loaded?
- Are any rules applied?
- Is my firewall script/program "running"?
The old iptables "service" was obsoleted by systemd. I think this project attempts to make a similar "service" available for Debian.
To see if the kernel modules are loaded:
# lsmod | grep ip_tables
To see the effective rules:
# iptables -L
or, if you want to see the NAT rules:
# iptables -t nat -L
In Debian there are a number of firewall packages that can set up the iptables rules for you. (I have not tried any of them, I usually roll out my own when I need it).
apt-cache search firewall | grep firewall | less