Is my guppy pregnant?

Is it female and over the age of 8 weeks? Yes, then she’s more than likely carrying fry.

Guppies are the master of evolution, they can reproduce in situations that most other species wouldn’t have a hope in hell of managing to carry on their bloodlines.

How do guppies breed?

Generally fish lay eggs. In the simplest forms, the eggs are laid by the female and the male then releases sperm to fertilise them. This has a couple of issues. The eggs are at risk of being eaten, the sperm and egg have to come together almost at random, and there has to be a male and female present at the same time.
Guppies, and other livebearers, get over all three of these, the male will insert his sperm into the female using a specially adapted anal fin, the eggs will then be fertilised internally, and the eggs will grow and develop before the female pushes them out as fully functional fish, bypassing the eggs stage, the fry stage, and being ready to swim off, feed and avoid predators from the moment they hit the outside world.
But here is the real advantage. The female guppy can take the sperm in, and store them. She can even influence which sperm is selected for her fry and maybe creating fry with the largest range of traits. Sperm may be stored for 9 months or more.

For guppies in the wild a female can swim to a new place, and then give birth to a whole new generation. In her lifetime she may lay thousands of fry, and they, in turn, will do the same. No wonder they’re an invasive species all over the world.

So you have a female guppy, and you don’t want fry.

Unless she was separated from all the males before she was about 4 weeks old, then she is likely retaining sperm. This means every 4 weeks you will have another batch of fry.

Are you saving them all?

You’ve got one adult female, and she lays 50 fry. Half of those are female and in 8 weeks they’ve all just had 50 fry. In the same time mum’s had another 2 lots of 50 fry, you’ve got well over 1000 guppies, and you might be finding that shops don’t want them. Other fish keepers aren’t willing to travel to take any more, and they’re still breeding.
Sadly euthanising most of the fry is the most humane option.

But I want to breed guppies.

Fantastic, and it can be a great aspect of the hobby. Step 1 is to choose adults with known genes. The British Livebearer Association is a good start. Look for specific colours and tail types. You will pay £20+ for a good pair, and a lot more for an excellent pair.
Now set up so that you can select the best fry. You might find that shops will take some of the fry, these will have known genes and the better or more unusual colours will allow them to sell them for a bit more.
Once you have some great fry consider showing them at a bench show. This will allow you to get feed back on your lines, you can use that feedback to improve your line.