I've used a few detectors in the past and they seem to work to a degree. Both of the ones I used would occasionally signal on nail holes that had no nails but did have a lot of iron oxide staining from corrosion. I mention that so if you get a signal where's there's no nail, that's par for the course.
I have three metal detectors that were designed to find metallic objects in the ground. They all have discriminator circuitry that allows you to refine the sensitivity of the detector to tune out specific metals or to ignore soil with high iron content. I have used them to test timber for embedded nails, staples, fence wire, etc. I always turn off all discrimination settings so as not to miss even a tiny piece of metal. As a Sawstop owner, I'd rather toss a suspect piece of wood, than buy a new brake and blade. I'm not certain how a Sawstop table saw would respond to a streak of iron oxide, left behind from a pulled nail.
In my experience, when one of my detectors thinks that it has detected a metal object, it is indeed an old nail. Most wood, especially when taken five or six feet above the ground, is going to be free of metals, but not always. I would think that most handheld metal detectors would do a good job of locating embedded metal objects, such as nails. A lot would depend on the thickness of the timber too. I'd probably run my Sawstop with the safety mechanism locked out and with an expendable blade.
