try pinging your router and something thru the modem. if the router is in any way stressed you will see the times in the packets rise, if you see the modem drop out and yet no change on the router its safe to say its COX.
you may even be able to ping the modem, most are 192.168.100.1, googles 8.8.8.8 is fairly close/low hops for me.
EDIT: windows ping -l or -ttl (cant remember) to ping long time