Look for some of these signs to start:
Decreased activity
slower motions
less vocal
sucking is weaker or slower
quieter
calmer
appears disinterested in toys/surroundings
eyes are less focused (i.e. looks around room)
eyelids are drooping
yawning
When they are edging on overtired, they may:
fuss
rub eyes
become irratable
cranky
Mine does several combinations of all. If she's rubbing her eyes and becoming irritable, she's really close to being overtired, and I may have missed the window. She does start looking aimlessly around the room, yawn, and beome a bit calmer. She also buries her head into my chest when I hold her (probably to block out light or other stimuli). If she's on the floor on her tummy playing, she will suddenly put her head down on the floor and start looking for her thumb. When I put her in bed, there is usually a little fussing, sometimes a little bit of crying, but it usually ends within 10 or 15 minutes, and then she's out. Occasionally I manage to get her down at exactly the right time, and she falls right to sleep, but that happens less than I'd like. She's a self soother, and does know how to fall asleep on her own, but sometimes she needs help.
*HUGS*
Sonya =P