Well, you can usually deduce some of it from what they like in food. Like my cats like tuna, but not salmon or whitefish or other common seafood for cat food, and they like chicken but not beef (and they prefer chopped bits in gravy over that pate shit Costco keeps pushing). So I just get them whatever has tuna in it. A lot of times, you can find samples on the websites, or you can ask friends with dogs if you can have a couple treats to test out on her. You might even be able to ask the people at Petsmart or Petco if they have a couple samples you can try. I don’t know that I’d worry about finding “the best” one, so much as one she loves. She probably won’t really train faster because she loves a treat more, but rather because the rules became clear and were consistent and you were patient and she had time.