Taking and recording the temperature.....

The most reliable sign of whelping is the female’s body temperature.  Normally her temperature will drop several degrees, from a normal 101.5 degrees to 98 degrees.  Once you see the temperature drop, you know to expect puppies within 12-24 hours. This 12-24 hour time period should give you more time to prepare for the birth of her puppies.  Although this method is almost fool proof, I have had a female that doesn’t drop in temperature before whelping.  I had to watch for other signs of active labor.
(1) Begin taking her temperature at your estimated day 56 of her cycle.
(2) Record temp in the Am and PM at the same time each day.
(3) Don't miss taking a temperature.
You might notice a big increase in temperature before it drops. Any temperature recorded lower than 98.4 indicates labor should begin within 12-24 hours.  If you dog goes beyond 24 hours and labor has not began, it’s time to call your veterinarian. 
It’s important that you use the correct thermometer.  Use a lubricated, oral stubby human thermometer.  I have also used oral human digital thermometers which worked just as well.  Always use the same thermometer when taking your bitch’s temperature.

How to Take your Dogs Temperature
(1) Cover the tip of thermometer with lubricant
(2) Have your bitch lay down, insert the bulb end into her rectum about one to two inches. 
(3) Keep the thermometer in for at least two minutes
(4) Remove thermometer and wipe with a tissue
(5) Read thermometer and record the temperature on your chart