In California, there generally is not a license for non-medical caregivers. This is different that a Certified Nursing Assistant or Certified Home Heatlh Aide.
Personality is most important: warmth, patience,
and adventurous. If your mother in law is forgetful or confused, that should not be frightening to the caregiver.
Special training in working with dementia helps. Sometimes instead of a license the person might have a certificate which shows what kind of training or how many hours.
If the person you want to hire does not have special training, but you like him or her, you might offer to pay for a course. Or you might ask her to attend a support group and pay the extra time. Call your local Alzheimer's association to find out what is offered in your area. Or check with Caring.com's local resources.