I didn't even know there was such a thing ( how can a 'rescue' not have a pet's best interest at heart??? )
It's really shocking how many animal abusers, brokers and collectors are out there, using these poor animals for their own personal agenda, or to make a quick buck. It's truly dispicable and they need to be exposed.
One way to do this is to email them and ask about their policies and procedures. If they say they will adopt an intact animal to you - even if they "give you 60 days to have it spayed/neutered" - that's one sign.
Another sign is if they usually have hard to find breeds around, lots of young Chis, Yorkies, "poo" type dogs, or other designer puppies.
Ask where these puppies come from, what vetting they've had and how much the adoption fee is, and if s/n is included in that fee. If the person gets angry and your questions or refuses to answer - that IS an answer.
NO reputable rescue will EVER adopt out an intact adult dog for ANY reason, and 60 days is plenty of time for someone to breed a dog!! Unless the rescuer moves in with the adopter, just how do they ensure the dog is not bred?