A good surgeon taking a decent history and palpating the patellas has a pretty good idea. Also, when they go in surgically, the presence of arthritis and degree of joint deterioration are also good indicators.
Dogs that are jumping up and down off furniture constantly are actually prime candidates for traumatic luxating patellas in their middle age years.
If you OFA patellas at a year old, chances are pretty durn good that the dog hasn't injured them yet unless a traumatic event is known, especially if you are a breeder who is conscious of this.
You can really damage their backs with jumping as well. Get ramps of steps for the furniture if you want your dog to get up and down on their own.
I don't think most breeders are evil people breeding unhealthy dogs purposefully. I do think health testing is a great tool to use to assure that things like injury years down the road doesn't affect you when you look at a pedigree later on. Also, you have a good idea of what you are dealing with and producing when you look at a pedigree and can actually look up the results of health tests. For the next generation of breeders, health testing can be a priceless tool. It certainly offers a great opportunity to look at the improvement of the breed.
