How do they figure out that Bella is pregnant in Breaking Dawn?

Answer 1

The special effects people create an fake pregnancy belly that the actress wears under her clothing. If an actress needs to appear as though she's going through different stages of pregnancy (i.e. 6 months to full term), they make different sized bellies.

Answer 2

Spoiler Alert!

First, Bella becomes violently sick after eating some chicken. She blames that on food poisoning, as she does not have any other idea.

Second, when she and Edward are sitting on the couch, she turns to kiss him and feels a very painful pinch/twinge in her abdomen. At this point, she doesn't know what's going on, but runs to the bathroom to be sick again, but can only make it to the sink. Again she doesn't know what's going on.

After the second or third time of being violently sick, she goes to find some Pepto Bismol, and sees a box of tampons in her toiletry bag. Openmouthed, she counts the days, and tells Edward. Alice calls, unsure of what she's seeing, and since Edward wouldn't pick up the phone, Bella demands to talk to Carlisle. After Bella says the word "pregant" Edward snaps up and asks to speak to Carlisle.

Carlisle then confirms that she is most likely pregnant.