As the name implies, Saddle Stitching is a hand stitching technique commonly used among saddle makers. The first leather saddle traces back to 365 AD and it can be argued that the saddle stitch has evolved since that time.
There are significant differences between a saddle stitch and a stitch done by a sewing machine. Technically speaking, a machine stitch uses two pieces of thread to "lock" the stitch in place, creating what is commonly known as a lockstitch. The upper thread pushes through the top of the material, such as leather or fabric, to the back of the material where the lower thread loops around the upper thread to "lock" the stitch. The upper thread is then pushed back to the top. The end result is the upper thread visible along the top of the material and the lower thread visible along the back of the material.
On the other hand, the saddle stitch only uses a single piece of thread and is a hand stitching technique. There are some sewing machines that claim to produce a saddle stitch, but are still lock stitching machines that are just strong enough to go through thick leather. There are certain techniques that help with the aesthetic of the saddle stitch, but a saddle stitch in its most basic form is where the ends of a single piece of thread passes each other through a single stitching hole and...that's it. Technically speaking, that is the basic concept of a saddle stitch.
While the saddle stitch in theory is quite simple, it is a time consuming process that produces a superior stitch. While a proficient machine stitcher can stitch a simple passport wallet in less than a minute, even the most experienced leather craftsman will probably take nearly forty-five minutes to one hour to hand punch the stitch holes and saddle stitch the same item.
The reward for the time and labor is a more durable stitch, possibly even the strongest stitch we know of. If the thread rips on a lockstitch, then the entire integrity of that stitching line is compromised. Have you ever found a loose thread on a shirt and ended up pulling out a foot or more of thread? Meanwhile, if a thread rips on a saddle stitch, then the only area that is compromised is at the rip. The integrity of the remaining stitches is still intact and even repairs can just be made at the effected areas rather than replacing the entire stitching line. Remember, saddle stitching is a technique that comes from horse saddles and a poorly made saddle can result in a deadly outcome.
But saddle stitching is not only used for saddle making, but also for handmade leather goods. Hermes, the high-end French luxury goods manufacturer, is one of the only well-known brands to still employ the technique. From Prada, Fendi, to Loewe, high-end designer handbags are stitched using machines despite being priced at nearly the same price as most people's mortgage or rent payments.
How does one know if an item is saddle stitched or not? First, one could assume 99.9% of leather goods aren't saddle stitched. That includes items sold by high-end department stores like Neiman Marcus and Net-A-Porter and especially applies to shopping mall brands like Madewell or Anthropologie. Even if the item is marketed as being handmade, it probably is not hand stitched. A craftsman simply using a sewing machine with their hands. For most brands, being "handmade" is convincing enough and so their isn't a big reason to lie about the item being hand stitched.
The most obvious indication of a hand sewn item is one that is being marketed as such. As previously mentioned, hand saddle stitching leather is such a laborious and time consuming method that only the most humble or forgetting craftsman will leave out that an item is hand stitched.
For more physical comparisons, saddle stitches are rarely as consistent as machine stitches. When setup properly, sewing machines are able to produce consistent thread tension. A leather worker that saddle stitches depends on feeling to get the right tension and to produce a perfectly consistent tension every single stitch may honestly be humanly impossible. Even the "best" leatherworker will probably produce a run of stitches that aesthetically do not look as consistent as a machine stitch. What is meant by consistency is things like the angle of the stitching or areas where the thread looks tighter or looser up against the leather. It could be said that hand stitching will always have more character and personality.
It use to be said that only angled stitches can be done by hand saddle stitching, but that is not true anymore. Probably in an attempt to produce Hermes looking stitching, sewing machines are now able to produce angled stitching holes rather than the traditional round stitching holes. At the same time, there are saddle stitchers who enjoy the look of round stitch hole. At this point, whether or not the stitching is straight or angled is not a good indication of a machine or hand stitched product.