You can use the rod method, but just know it won't be accurate since the spark plug is at an angle. All you'll be able to tell is if the stroke is greater than 3.31" as previously mentioned.
That's a valid point and still satisfies most of the question of "Is it a stroked motor". I supposed the bore scope might reveal a piston part number as well which would be the best indicator I think. Wouldn't be much of a chore to put a rod through the spark plug hole of an unmounted head and get a "general" idea how far down the hole the rod tip ends up opposite the spark plug hole. Add that to the actual measurement and that's about as close as you might get.