All that is needed is a male guppy in with her.
The Gravid female will have the dark "gravid spot" in front of her vent. I can however assure you that if there has been a male in the same tank with a female guppy for any reasonable length of time she will be gravid.
The female guppy will develop a dark spot on the side of her belly during pregnancy. As the pregnancy progresses, the spot will grow darker.
The female gets a deeper tummy and there is a dark (gravid) spot by her anal fin.
No.
With a guppy you need to look at the gravid spot. This is a dark spot in the fish's belly. A large gravid spot indicates the fish is likely to be pregnant. You can tell how pregnant the fish is by how large the spot is. Sorry I have no experience with other species.
No, unless a) the female fish got pregnant before the male died, or b) you buy a new male guppy and put him in with the female.
A guppy fish is a live bearing fish, but the female generally does not take care of her young, and will eat them.
Female guppies are larger and plainer. The male is distinctive for his long, flowing, colourful tail and generally brilliant colours.
The signs of guppy breeding is when the males (the more colorful fish) start chasing the female guppies (the duller ones). The female guppies should appear to become fatter, signifying that the fish is pregnant.
Quite often a male guppy will nip at the female if she is pregnant; this can stress the female and make her ill.You can place the female in a birthing cage early on if this is a problem, but pay extra attention to the cleaning of the birthing cage daily.
Under normal circumstances every female guppy you come across will be gravid (pregnant). My advice is don't worry about it she will be pregnant.
When your female guppy is pregnant you can't tell by it's size. You have to look at it's gravid spot. Which is right behind their stomach and the tail. It's right around the butt of the guppy. It takes about a month for the babies to be born. Plus you can not stress out your the pregnant fish.