Welcome to the forums!
If you don't mind, I will start by asking a few questions since this seems to be your first time in trying to breed budgies:
For how long have you had your budgie couple?
Are you relatively new to owning budgies?
Have your budgies been properly conditioned to breed?
How old are your budgies?
Before taking the breeding route and in order to do so more safely, a person should have a very good grasp of the species (this comes with real life experience in budgie ownership) and to do the required research into the subject.
The decision to breed comes with a whole lot of responsibility and commitment, the lives of the breeding pair(s) and the chicks are depending on you and the response you give if/when faced with adversity.
Things like being able to tell if your breeding pair is in good health and top physical condition to go through breeding; when a hen is expecting an egg or if she is showing the first signs of being egg bound; when a chick is having developmental problems, not being fed or showing signs of dehydration;
when there is aggression, abandonment and neglect of the chicks and it's solely up to you to feed and raise the chicks. The ability to detect early on and solve these issues can truly make a difference on the outcome of your breeding journey.
Please read this link: A heartfelt plea to forum members new and old
Now to answer your question, you can see if your budgie is carrying eggs by looking at the vent area and see if there is some swelling there.
Another good indicator is the poop which will be massive in size.
If possible, post a photo showing your budgie's vent area so we have a better idea of what is going on with her.