a pregnant female guppy will obviously look fatter than usual and also have a little black dot near the vent which is called a 'gravid spot' this is supposedly the developing eyes of the fry but who knows the females stomach will be more squared off, do you have a live bearer net or box ?
Did she share a tank with males?
If she has been in contacts with males in the last six months you can almost guarantee that she is pregnant because female guppies have the ability to store sperm so they can continue having fry even when they haven't recently been in contact with males.
Have a plan of action for when she is bigger (breeder net, fry tank etc.) and keep an eye on her