In the chao gardens, a green symbol will appear above a chao's head. Here is a simple guide to what they mean (most of them are obvious, though).

A Heart- He's happy.
A Swirl- He's upset. Make him happy.
A ?- He is busy deciding what to do next.
An !- surprised. could be good or bad.
X-ed eyes- very ticked!
Almost closed eyes- Very happy!
Halfed closed eyes- sleepy
waving arms- hug it!
flinging arms- temper temper