When my son was a lot younger (he's 6 months now) I was soooo worried about him being OT I would swaddle him up at the first, what I thought was, tired cue. He would yawn and that wasnt his tired cue, he was bored! At 8 weeks I would suggest changing the environment once or twice to make sure they are actually tired and going from there.
I have never read of signs of being undertired other than not falling asleep and not acting tired. My sons cues are eye rubbing and his little eyebrows get red when he's tired. I would assume as well that they change their cues as they get older. As with today, Wesley rubbed his eyes quite a bit and I put him down, an hour later he still wasnt asleep. The day way shot and he went to bed at 6:15. Try again tomorrow!!
I hope that helps a small bit, good luck!