I don't know about other Shih-Tzus, but when it's time to rest, my boys have some peculiar habits.
One thing I have noticed is their little eyes will start closing and their head will begin to droop even while they're sitting up. That's when you know they're really tired.
Other than that they each have a ritual before bed. Bruiser will lie on the floor with his back legs out under the kitchen table. That's phase one of sleepy time. After dozing for a while, he'll move into the family room on his cushions, usually with the "toy du jour".
Rocki, on the other hand, will start off by the garage door around the corner. After catching a couple of zzz's, he moved into the kitchen flush against the wall. He naps for a bit, and then moves over to his favorite cushion seen in the picture. His favorite position for sleep is on his back with his paws in the air and his body touching the wall. Bruiser, on the other hand, likes to curl into a little ball and as the night goes on, his head is on his cushions and his wheels are in the air as well.
The boys like routine and they have found ones that suit them. You could actually set your clocks to them. At exactly 8:45 they start getting into position. By 9:00, they are in one of their various stages of sleep. Of course, early bedtime means early rising, and we have had to adjust our sleep schedules to accommodate it!