My DD has always had clear sleepy cues, ie:rubbing eyes etc
As for DS, well, as a baby I would have no clue whether he was tired or not, and it came to me that as a pp said, he would become more active when OT, like hyperactive!
Does your lo not show any signs such as just generally going quiet? Just one to throw in as my friend's lo always just goes quiet when strating to get tired, no yawning or fussing, just quiet