Is there an easy (external) way to tell if a female cat has been spayed? A neighbor is considering adopting an abandoned stray and I'm trying to help them determine if the cat has already been spayed.
When cats are desexed as part of a "catch and release" program (to manage ferals), the medical staff usually clip the tip of one ear (use of this practice varies with providers). So, if the cat is missing an ear tip, she is probably spayed.
You can also look for a scar from a spay. If there is a scar, that's a good indication that she has been spayed. It's not a 100% indicator (she may have had abdominal surgery and not a spay), but it's highly likely.
According to Long Beach Animal Hospital, for a midline spay (common in the US) the incision is usually made near the cat's belly button and located at the "linea alba, and is the area where the abdominal muscles meet". There are (graphic) pictures of the surgery at the link.
In some locations (such as the UK) flank spays are more common than midline (belly) spaying. If you are in one of these locations, you should check her flank instead of her belly for scars. The most common location for a flank spay scar seems to be where the leg meets the belly, but I recommend a google image search for "flank spay" to get an idea of how widely the position can vary.
During surgery, the vet will try to minimize the length of the incision so that healing is not as difficult. The result (in a healed cat) is that the scar will be small and may be difficult to find. To see the skin you may need to shave her (which can be difficult for a stray you are trying to build trust with!).
If she goes into heat (i.e. estrus), she has not been spayed. VCA Hospitals gives the following information about symptoms of cat estrus:
It is not common to observe vaginal bleeding from a cat in heat. The most notable signs of estrus in cats are behavioral. Most cats become very affectionate, even demanding; they persistently rub against people or objects such as furniture, rubbing against their owners and constantly demanding attention. They roll on the floor. When stroked along the back or spine, they raise their rear quarters into the air and tread with the back legs. They also become very vocal. These behavior changes often become annoying to owners, and sometimes owners think their cat has some unusual illness.
Some female cats will urinate more frequently or may even spray urine on vertical objects ("marking") when they are in heat. The urine contains both pheromones and hormones, both of which act as signals of her reproductive status to other cats. This is the reason that queens in heat attract intact male cats. In some cases, this may be the first indication that a young cat has reached puberty.
Tomcats that have never been seen before in the yard or neighborhood will appear and may attempt to enter the house to mate with the female, or may spray your house with urine to mark 'their territory'.
When in doubt, ask your vet
Your vet is the best source of information about the health status of your new pet, and you should always take a new family member in to the vet within the first 2-3 weeks of ownership for an initial assessment. That allows the vet to see your pet in a normal, healthy state so that if you have to bring her in for an illness later the vet has a baseline to compare. An initial visit is also a good time to test a stray cat for diseases such as FIV or FeLV, and get age/area appropriate vaccinations.
Additionally, when considering adopting a stray cat it is good practice to have the cat scanned for a microchip. Some microchip databases track spay/neuter status, and the cat's owners may be looking for it. Most vet offices have a scanner and can perform this check for you.