What are the signs to tell if my dog is pregnant before spaying?

There is no sure way to know if a pet is pregnant through physical examination alone, especially in the 1st trimester. Most of the time you can palpate moving puppies in an enlarged belly during the 2nd trimester, but sometimes this can prove challenging in overweight or large breed dogs.
Some common tell-tale signs that suggest you may have a pregnant dog are: swollen abdomen, weight gain, and enlarged mammary glands. Then you can confirm your pet is pregnant through x-ray or ultrasound (to detect fetal heartbeats). The gestation period (time that a pet is pregnant) in dogs is 58-72 days, an on average around 63 days.
It is a good idea to get Martina spayed if she isn't pregnant, to not only prevent future pregnancies, but to also prevent other disease processes like Pyometra (acute infected uterus) and mammary cancer from forming later on in her life.