Your state (New York) has pretty weak puppy lemon laws so you really must rely on the breeder.
New York Pet Lemon Law
A reputable breeder stands behind the health of her puppies. I have seen contracts with a five year health guarantee. A member on another forum I belong to Maltese was diagnosed with a liver shunt at about six months old. The breeder paid for the entire surgery.
Luxating patellas are a little more complicated as they can be genetic or caused by injury. The general cutoff is about a year. If diagnosed before the first birthday they are assumed to be congenital. After that, they are usually from injury like jumping on and off furniture. For that reason, it's very important to have your vet check your puppy's knees at the very first vet visit.
As Mardelin frequently says, "buy your breeder first".