Watch out for other obvious behaviour too.

Our bitches often used to really empty their bowels not long before coming into the labour. Watch out for small amounts of it, sometimes with a bit of mucus where they are really trying to clear themselves out.

Nearer the time, they get restless, might start rushing around to find a place to have them and they'll rush anywhere except the area you made for them to have the pups. Our Mist used to be all over the garden trying to find another den, haha. During this time, it was almost impossible to get her to settle in her whelping area. She'd insist on rushing around everywhere before finally settling.
They also start shaking, panting and sometimes whining like they are excited. They also tend to go off their food about 24 hours beforehand but our Jess didn't the first time she had pups.

Good luck. I hope it all goes smoothly.