Take her to the vet for her vaccs, and have the vet take a look. It is very hard to tell sometimes, and even some vets can't tell (some open up dogs and find they are already spayed after not finding a scar, and others think they see a scar, than a month later the dog goes into heat (or worse, delivers some unexpected puppies!). I've been somewhat of a vet tech for 14 years (I'm not an "official" vet tech - I just do vaccinations, give fluids and meds, etc, for the animals in our all-breed rescue who ALL get spayed/neutered), and I rarely even try to spot a spay scar - I just hand them to the vet.
There can be different bumps and scars on an unspayed dog's tummy, such as their "belly button", or a scar from an umbilical hernia surgery when no spay was performed. On the flip side, a spay can be performed by a vet who uses a very small incision, so it can be missed.
I woke up about 2 minutes ago, so if this doesn't make sense, just read the first sentence and ignore the rest, lol!
Congratulations on your new dog!
Edit: I didn't mean to repeat what Tara said on the vet - I hadn't read it yet. So ditto Tara! =)