Look for a gravid spot right behind the stomache of a female. If it's very dark and noticeable, it's likely extremely gravid. Also, check for a "squaring" off of the stomache of females. Gestation is usually about 28 days, but can range from 21 days to 40 days, depending on a couple of factors like genetics, temperature, etc.
hmmm if all the guppies are gravid... and almost ready to give birth [been hanging at the bottom alot last couple days] then why does my male still court them?