You will know for sure, she will be irritable and there will be bloody discharge. I would spay her! It is better for the dog and a lot easier for you. Especially if you don't intend on breeding or showing her.
Even if she is an inside dog, crap happens. She could get out of the house, or get away from you at a dog park. It would only take one such incident to end up with puppies you don't want. Sure the chances are low, but it's better to be safe than sorry, right? Not to mention the health benefits of spaying.
If cost is an issue, check your local animal shelter for low cost spaying. Mine offers a voucher for people who can't afford to spay or neuter their dogs and cats. Your vet might know about resources too.
Luna+Ichi