It's really very nice to hear that you want to watch out for delivery implications. Because cats have multiple births and each kitten will have their own amniotic sac and placenta, there won't be a water-break like with a human.
A typical cat pregnancy will last 63 to 69 days, depending on the accuracy of the human's calculations of actual impregnation. Before becoming an animal rescue facility, we owned a cattery with registered Siamese varieties. Our queens almost always birthed on the 63rd day. If a pregnant cat goes beyond 65 days - I'd schedule a vet visit. We also made at least three vet visits during the pregnancy to monitor health (of queen and kittens), and to know when delivery would be likely. This is of utmost help.
This link explores possible complications and other details http://www.sniksnak.com/cathealth/howto15.html
About a week before the cat is due, start taking the temp at the same time, every other day and record it. When it's about 2-3 days prior to what your calculations indicate is delivery time, take the temp every morning and evening. A drop of approximately 1 degree (or slightly more) will often mean that delivery is impending, usually within 24 hours.
Cats absolutely need privacy, quiet and a sense of safety and security when delivering. An audience, noise or well meaning humans trying to "help" can stress a cat out to the point of their moving the newborn kittens and putting them in danger; becoming aggressive or cannibalistic toward the kittens or abandoning the kittens. Rarely does a cat need ‘help' - especially if a check up is undertaken at the vet before delivery, confirming overall health of mom and babies.
Leaving mom and babies alone for the 24-48 hours after birth is essential. Do not attempt to change the bedding at this time. Do not bring visitors in to see the kittens. No one should handle the little ones until their eyes open, about 7-10 days and even then, limited handling is recommended as they are still very fragile and mom may be uncomfortable with the interference.
If contractions last an hour without producing a kitten, this is an emergency situation. Quickly and carefully get your cat and any kittens she may have already had, to your vet.
If you're sure there's a placenta being retained after your cat has finished giving birth, she needs to be seen by your vet quickly. Retained placentas cause uterine infections that are often life threatening.
If a kitten does not emerge from the birth canal in 10 minutes, this is an emergency and you need to be prepared to intervene. Intervention would mean getting her to a vet, no matter what time of the day or night. You need to be ready to do this.
Good luck and congratulations on the impending births.
They don't have signs that you might notice. They have behaviors that would indicate they're possibly close.
If she's found a quiet, dark, safe place to "nest" for example, and she goes there a lot and spends a great deal of time there, it's a behavior that may mean delivery is just a few days away.
The regular temperature monitoring is a physical indication of labor impending. It's outlined above.
Some queens will have a mucous showing within 24 hours of beginning active labor, but people don't always catch this because the cat cleans it up pretty quickly.
The reason cats and many other animals don't show signs of impending labor like humans do is because in the wild it would make them targets of predators, who just by nature would sense a weakness or vulnerability.
Once your cat is in active labor you'll notice her abdominal area hardening and then a big wave-like pushing occur (once you see it you won't be able to mistake it for anything else). She might make some noises while it's going on, maybe crying out a bit or moaning. It's pretty important to leave her alone though. If she seems to trust you, then offering her comfort and reassurance by speaking in a low, gentle voice might help. Though you might want to pet her, it could distract her from the task at hand and prolong her labor so just the vocal coaching is suggested.
I hope it all goes well,