I can't help you with what you can check at home, other than pull the pan and look at the fluid and for any sediment. The shop might be able to run a pressure check on it, but will likely tell you that you're looking at tearing it down for a rebuild to be sure. Our local independent gets $1500 to $1800, but that's R&R too so you might save a bit carrying it in. For a shop I'd visit a couple of independent auto repair shops that you know have good reputation, and ask them where they send tranny's for work. Most of them don't do them in house, and they use the best independent shop in the area for their customer's cars. If you tell them you have a tranny out for a project they should be give the recommendation.