Although not scientifically proven, a temperament/scent test can help predict a puppy's potential as a service dog. To give you an idea of what I see when I perform this test on a pup (or a litter of pups), here is a copy of Stella's evaluation.
Stella has been in her forever home - with her type 1 diabetic - since she was eight weeks old. Scent training started right away, as Stella encouraged to be part of her diabetic's "checking" process. We are seeing signs that Stella is becoming aware of her diabetic's fluctuating blood sugar and reacting to them. Eventually scent recognition will become a trained alert.
For now, Stella has already finished her first obedience class and is going more and more places for public access training. She spent two days at my house recently for a "board and train" - while her people enjoyed a family wedding. Below: my chocolate Lab Murphy takes up the rear with a down/stay, with Stella in the middle demonstrating a puppy sit/stay, and my old girl (she's about to turn 13) up front with her stand/stay.
Great dogs, all!