If you check out their adoption process, this should give you a rough idea. Call them and ask about their “return policy.” Rescues usually have the “forever home” in mind, whereas a shelter may just say “if the dog doesn’t work out, just bring them back.” Also, there are many internet groups dealing with rescue. Sign up for the breed groups you are interested in. You can get a good idea who is “reputable” within a few weeks. These are active rescuers and they are full of advice and know who’s “naughty or nice.” Those who are actively rescuing dogs are more likely to be reputable.