How can you say that your baby is a happy baby?

I have three children and I feel that one can one's baby is happy when you look into the baby's eyes and there is spark, and that spark you will see in happy babies. I mean, other babies too can have that spark, but happy baby will establish that happy spark with the movements of his legs and hands. Lol! I do not know whether you got my theory or not but all I meant was that you can see the happiness in the baby's eyes. My third baby was a really happy baby. I mean all the time. Other two had their moments and hours of happiness. ;p