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About 15-18 days into the pregnancy, a queen's nipples will "pink up," or become red and enlarged. Her breasts may enlarge, and she may express a milky fluid. Enlarged nipples are also signs of being in heat, so bear in mind that growing nipples are not exclusively indicative of pregnancy. From the side, pregnant cats frequently look somewhat swaybacked, with a slightly round and bulging abdomen. Many female cats assume this burro shape later in pregnancy. If your cat is merely overweight, she'll be heavier all over, including her neck and legs, and not just in her abdomen. A few days before she'll give birth, your cat will begin showing nesting behaviors as she prepares for the arrival of her litter. Your cat may go to a quiet place like a closet and start arranging blankets, towels, or other fabric to create a place to give birth to her kittens. If you notice nesting behavior and you hadn't previously realized your cat was pregnant, take your cat to the vet as soon as possible for a prenatal checkup.
Check for enlarged nipples. Look for a characteristic "burro" shape. Notice any nesting behavior.