It's important to be aware of your baby's sleep cues so that you can put him to bed with perfectly timing--not too early and not too late. This will help him fall asleep more easily and stay asleep for longer periods of time.
Every baby is different and one baby's sleepy signs and overtired signs will differ from one another, but this list should help you
out while you are trying to figure out your individual baby's sleep signs.
Common Sleep Cues
Slight quieting, a lull in being busy, rooting or wanting to nurse, asking for a bottle, pacifier or lovey, a slight staring off, decreased activity, slower motion, less vocal, sucking is weaker or slower, quieter, calmer, disinterested in surroundings, eyes less focused, eyelids dropping, yawning, less movement of arms and legs, eyes that are not as sparkling or eyes that look "glazed over", eyelids that droops a little, less intense staring, less socially responsive smiling, less engaging, rubbing eyes, irritability, pulling hair, thumb sucking, bating at ears and crankiness.
A baby (especially a very young one) can go from sleepy to overtired very quickly so you need to act quick when you first notice your baby's sleepy cues. Ideally you will already be in the process of getting baby ready for sleep when you first notice sleep cues to prevent overtiredness. This is where having some sort of routine can be really helpful because you know when to anticipate sleep cues/tiredness.
Also, sleep cues can change over time, and it isn't too uncommon for them to disappear when your child gets older (although their overtired signs will probably still exist but may be more subtle). For example, my son Joshua would yawn all day when he was a newborn, not just when he was tired, so I wasn't able to use this as a sleep cue. When he was a few months old he would yawn when he was ready for a nap so this was useful as a sleep cue. At 12 months he would only yawn when he was extremely overtired or after I had already initiated his pre-sleep routine making the "yawn" no longer useful as a sleep cue. Some newborns also appear to take several weeks before they have notable sleep cues while other babies appear to never get any at all (although someone as experienced with babies as TBW might not agree with this).