I consider myself truly hungry when I experience hunger pains and when I have that empty feeling in my stomach.
For me the true battle is resisting my appetite and cravings -- this is a strong desire to eat something because of how it tastes or makes you feel when you eat it.
So if I think I'm feeling hungry, I'll do a mental evaluation and ask myself if I'm experiencing appetite or true hunger.
You can always do the scientific thing and count your calories and monitor your measurements and body weight if you're unsure.