I could be wrong, but I think the only way 'intent' would enter into the situation is if you had the complete pistol, with a stock lying around without a rifle length upper available for it (intent to construct an SBR). If you don't have a rifle length upper lying around, make sure you don't have a stock lying around, either.
As for how they verify it, that's simple. They require you to prove it. Just make sure that the word "rifle" doesn't appear on any of the paperwork connected to the stripped lower you purchased for the build, assemble it as a pistol, take pictures to show that it was assembled as a pistol, and you should be in the clear. I also keep a copy of the paperwork (from the gunstore), showing that it was sold to me as a stripped pistol lower, inside the pistol grip - in case some curious officer sees it and has any questions about it's pistol configuration.