First off, I have to recommend the video above about the 5 baby words, this helped me out a ton with babies #1 and 3. It makes a big difference to get the DVD because then you can hear it better and for longer, helping you to be even more in tune to their sound cues.
This only helped a little with babies #2 and 4 because neither have been as verbal as infants. #2 was my easiest because she naturally fell into her own schedule and was so consistent that I could tell the time by her and if I ever wasn't sure I'd just look at the time. #4 has been easy mostly because of everything I learned from my previous 3, he doesn't make the sounds as much but he gives physical clues that I recognized from the others.
If they are hungry the first sign is usually a slight sucking sound or they stick out their tongue.
#1 only hours old "feed me, Mama"
If they have intestinal gas they may grunt or stretch and kick their legs. I've heard a lot of ways to help them with this but I've found that the best way is usually to lay them on their backs and hold their knees up to their tummies and rock them side to side, the rocking is just for comforting. Some babies prefer being held football style and patting their back or bum while you walk around or holding them tummy down across your lap and patting their bum.
If they are tired or sleepy then they start to make lopsided expressions like one eye might droop or their smile might be crooked. They also might get a worried expression, which really just means that they are getting over stimulated but that's often from getting tired.
He may be smiling but he's getting tired.
If they get fussy out of the blue, hardly any signs before they are full on fussy or crying, then it almost always means that they are uncomfortable and most likely need their diaper changed. It can also mean that they are hot, cold, or something else is making them uncomfortable.
All babies are different, some are self soothers while others need a pacifier or swaddling, some fall into a natural schedule while others are more unpredictable, some prefer their own space while others love to be held. I've had each one of those and seen many more and while they are all different they all have basic needs and basic ways of letting you know which one it is. I hope that you've found these tips to be helpful, they sure made my life easier.