Look for:
dialated pupils,
heavy breathing,
no interest in food/water,
discharge from the vulva; the plug dropping (water breaking)
licking at rear end,
staring at rear end in confusion,
contractions; a pulsing after the last ribs,
I believe cats pant also during whelping,
and KITTENS OF COURSE!
Just to name a few things off the top of my head.