I would say it depends on the stature of your opponent.
If you're fighting a chub kind of guy, wouldn't hurt to check the waist, hips, constant scanning.
If you're opponent is ripped, definitely watch the chest. Anything coming can be signified by a flex of the chest muscles (except maybe kicks although you would still see the chest flex to stabilize the body before the kick.)
I'm pretty sure most boxers are taught to watch the chest for muscle movement... could be wrong though. Watching the eyes is a stare-down pre-fight thing only