I recommend that you take your new cat to a vet and get her registered as a patient there. She's bound to need a health check and other treatment at some point, so finding a vet now will save you the worry of finding one when she's sick. A vet will be able to check for a spay scar. In some countries cats are tattooed to show they've been spayed, which will make it easier for the vet to confirm.
If you can get details of her original vet, as Verna Davis suggested, ask for details on vaccinations and blood work too. All very useful information for your new vet to have.
here - cats often get tattooed - you would be looking for a small line or dot - usually in blue or black close to the site of the spay scar ( which can be tiny if they got spayed as kittens) ... I would get her vet checked anyways and update vaccinations unless you have paperwork that proves that she had shots.