I don't necessarily have too much help for you b/c I have the same problem. My DD is 14 wks and she'll start to fuss from being OT and will be smiling at the same time...I just start the 4 S's with the first yawn. Sometimes my DD will get really quiet and lose interest in toys or w/e we're doing at the time and/or she might get a glazed look in her eyes but most of the time, neither of these are the case. Lately, I've noticed that she'll start sucking on her fingers towards the end of A-time so I take that as a cue too. But for the most part, I'll just start wind down 45 min-1hr in A-time and I'll get lucky and catch the window. And lately, I've been revising her activities b/c what you do in your A-time can actually be why she's overstimulated. Hopefully, I've helped a little. I know exactly how you feel though so Hang in there!...I'm going through it too.