I have interacted on the ISTP sanctum enough to know at least that I am probably not one. It has helped me with identifying them in real life I believe. I probably have a sensor bias due to my line of work; I have a tougher time identifying N-doms.
Pursuant to our discussion, I'd say that a good frame of mind to approach this difficult problem with, is that you are looking for an extroverted perception function, meaning you are ignoring S/N definitions and focusing first on generalized perception.
Once you are able to identify your perceptual processes as distinctly extroverted and introverted, respectively, you can then heuristically apply common S/N differentiation and you might surprise yourself with a new type altogether.