Here's a link to a great story on San Francisco's KTVU recently, a special report on what can go wrong if a caregiver is not vetted properly. This is an extreme case, as TV stations are wont to feature. . But it's well-intentioned and points up a problem that is only going to grow as the population ages.
Our advice is to pick a good geriatric care manager first, who will know which are the good caregiver providers in your area, and which aren't.
But if you can't do that, at least find a caregiver agency that employs its workers--NOT one that uses them on a subcontractor basis.