By no means am I the baby whisperer, but I'd like to think that I am pretty good at reading cues babies give off to let us know their needs. For example, sucking on their fingers indicates that they are hungry whereas having irregular breathing can indicate they are drowsy. I think that this is important for new parents to understand, especially when it comes to understanding why the baby is crying. Afterall, not every tear shed from the baby means they neeed a bottle!
Instead of re-writing every signal babies tend to give off, I am providing links to a couple of sites that do a good job of explaining this already