how to recognize that a fish is pregnant?

The goldfish will become fatter around the stomach, however, it isn't pregnant. Gold fish release eggs into the water that males fertilize. If you want the baby goldfish, you should seperate the eggs into a small aquarium since the fish will eat their owns eggs.

This usually occurs during spring, but if you slowly raise the temperature of your aquarium during winter, they will spawn as the temperature rises.

As Goldfish are egg scatterers, technically they don't go through pregnancy as females will only hold unfertilised eggs. They will look somewhat bloated when they're holding eggs, they should also be active and healthy during this. Bloating can also indicate other problems however.

See here on how to sex Goldfish:

http://www.bristol-aquarists.org.uk/goldfish/info/...

Yup. Pregnant carp get very fat-looking quite fast.

I don't know in the early stages but when if it can be seen than you could see it's stomach near the "tail" fat and really soft to touch..

There isn't a very specific way to tell. The only way to tell is if shes fat and maybe acting weird.