Here are the rules of thumb for whether you're responding to physical hunger or feeding a deeper emotional need.  Taken from the Mindless Eating book by Brian Wansink.

Physical Hunger:                                                     Emotional Hunger:

• Builds gradually                                             Develops Suddenly
• Strikes below the neck                                 Above the neck
• Occurs several hours after a meal          Unrelated to time            
• Goes away when full                                   Persists despite fullness
• Eating leads to feeling of Satisfaction     Eating leads to guilt
•                                                                             and shame