I'd like to think I've gotten pretty good at deciphering my baby's cries and moods. Hungry = she sucks her hands like crazy. Tired = whimpers and usually slows down (she never stops bicycling her legs, but it at least gets slower when she's sleepy). Gassy = she's even squirmier than usual.
I can also tell a lot by her face. For instance:
Overall, Libby has an incredibly good temperament. She's usually only fussy around feeding times and that generally has to do with being a) hungry, b) gassy, or c) both. Otherwise, she's all smiles.
Hooray for happy babies!