Try Oskar Andreasson 's tutorial:
It's very helpfull and you will find help on loging,too
In your case:
iptables -A POSTROUTING -s 10.0.0.0/255.255.255.0 -j LOG -log-prefix "Text to read in log"
iptables -A POSTROUTING -s 10.0.0.0/255.255.255.0 -j MASQUERADE
So, first log,and after do what ever you want with those packets.