Hi, good question. It has been my experience with them that a female is pregnant when you can see an enlarged abdomen. Later in the pregnancy, you will be able to see a dark/black spot near the anus which will be the eyes of the offspring. I am not sure of the amount of time it takes before you can see these things.
I do not know if the mom will eat them right away, but any other guppies and fish you may have in the tank most definitely will. If you have a heavily planted tank they have a chance. Stores also sell breeding traps that separate the fry from the mom. Personally I do not like them. The small confined area seems to stress the mom to the point that she will not even eat.
Guppies are a rewarding and exciting fish to keep. I wish you the best of luck with them.