Uterine enlargement can be palpated between 25 and 30 days.  Radiographs (x-rays) will show uterine enlargement between 21 and 42 days.  Ultrasound is earliest and most accurate diagnosis of pregnancy.  Ultrasound can detect pregnancy as early as 20-22 days but more consistently at 26 days.  The fetal skeletons calcify at 45 days and can be seen on radiographs.  Radiographs at 45 days or later are the most accurate way of determining how many puppies are being carried-though not 100% accurate.
Good Luck with the puppies!