I always scan my bitches - £20 to our local sheep scanner is well worth it. Archie is 100% accurate, and I then know exactly how many pups to expect and if they don't all arrive I know the vet may be needed.
Approximately 24 hours before whepling her temperature will drop by one degree. The average temperature of a healthy dog is 101 °F or 38 °C, however, the normal temperature of a healthy dog may range from 99 °F to 102.5 °F (37.2 °C–39.2 °C), so check it a few times for teh next week to get an average for her. I use a digital thermometer - faster and accurate.
She is likely to not eat a few hours before, and she will start to nest, and start panting. If she starts to push, it can go on for about an hour before any signs of a puppy but once it gets to an hour phone your vet for advice (and a little moral support)
Have you got a proper whelping box and all the necessary equipment? You'll get a list on here
http://www.phi-vestavia.com/whelplist.htmAnd don't think you won't need this stuff - you JUST might so it's better to have it then not, and be able to save one or more pups.
Lastly - Alert your vet now, you might not need him/her but you might - like I did last time. Saw dark red gunge appearing from vulva, guessed it was a detached placents. Called my vet, took her straight to the surgery and they were all already prepared for a Caesar and puppy reviving - an incubator already warmed up, and a box (with surgical gloves filled with hot water) ready to pop them into when resussed.
You might be lucky and not need all of this but better to be prepared.