Showing my age now...long before the 'non-contact' age, internet, cell phones, cordless landlines, and a whole lot of stuff........
Locate a 'ground' somewhere within the building, or if that's not possible, a known neutral point. even if you have to go back to the service entrance. Connect a suitable length of #12 stranded THHN or equal to said point & stretch it out to the location you want to test. You then have to skin a test point on the K&T conductors and use whatever tester (wiggy, digital, etc).
I did not say it was easy or quick; but it works. If I had to do it today, I'd pull out the non-contact, I have an Ideal, Gardner Bender & a TIF. Any of those are available thru WWGrainger, most electrical supply houses and the big boxes. Look in Tools for Electricians in the right hand border ads right here also, you may find one.