The protocol for veterinarians treating feral cats is relatively standard since ferals are healthy and require only sterilization, vaccination and worming. The hundreds of clinics across the country that treat ferals may differ only slightly in their procedures. Contrary to common assumptions about feral cats, there is no evidence to suggest that the majority of feral cats are in poor health or are any more likely to be infected with FeLV or FIV than domesticated cats. Certainly, kitten mortality is high - at least half of all feral kittens born never reach adulthood. If they do, their immune systems are able to fight off most viral and bacterial infections.
Available data indicate that feral cats have a very similar rate of infection.
The answer depends on a number of considerations. The decision to test or not should be based both on the goals of the humane management program and the welfare of the individual cats. The enormous tragedy of overpopulation is what prompted ACA and similar groups and individuals all over the country to start large-scale sterilization programs for feral cats. Controlling breeding through non-lethal means is and has always been the overriding goal of all of these programs. Before making a decision about testing, it is important to assess the cats you are planning to trap and formulate a management plan.