One of my "jobs" now at the assistance dog organization I volunteer for is to evaluate the new dogs that come into the program, including rescues. One of the first dogs on my list was a 1 1/2 yr. old boxer/lab mix now called Betty. She was scheduled to be euthanized until someone called us about her.
Betty did fairly well on the temperament testing portion of our evaluation. She enjoyed being handled, noises didn't startle her, she was confident and not food or toy aggressive. On the trainability section, she did just OK. She wasn't very food motivated and she was more interested in other dogs than in humans. We need service dogs that want to work and please and who love food.
I'm happy to say that Betty and I have found her weakness when it comes to food: dried chicken breast jerky from Sam's Club. No one knows her background, but I'd bet she's never had chicken breast that didn't come from a garbage can.
Betty has also learned to trust humans. She bops over to me when I or the other trainers call her. She's not like your average lab/retriever who loves to work. But, I suspect that will improve a bit over time.
I've never been a rescuer of anything. But, I'm pretty proud of Betty. She's going to have a great life -- whether she makes it through the program, or not. There are about 4 people on the waiting list to adopt her if she flunks out. So either way, her future looks brighter than it did before I met her.