How do we know that voltmeters are accurate?
These days, you build a primary voltage standard from a bunch of Josephson junctions and a microwave source.  That generates a voltage that's dependent only on the defining constants of the International System of Units (SI base units).
As a more economical alternative, you send your voltmeter to a lab that compares it against a voltage standard that's traceable back to a primary voltage standard.  In the US, that primary voltage standard is probably at NIST.
Basically, every physical quantity can be mapped back to a physical constant that is defined, rather than measured.  Seven of 'em (read the Wikipedia article) are base units; the rest are derived.  The volt, in particular, defined as the amount of electromotive force necessary to impart exactly one Joule on one Coulomb of charge.  In SI base units, \$\mathrm{1V = \frac{kg\cdot m^2}{A \cdot s^3}}\$.  So just build any old dingus that lets you generate a volt as long as you know what those four quantities are, and you're done!
As of May 20, 2019, all of these base units can, in theory, be reconstructed from first principles (i.e., the second is defined by a number of oscillations of a cesium maser, a meter is defined from the second and the speed of light, etc.).  Ultimately all you need is a one-page reference guide, an astonishingly deep understanding of physics and metrology, and a staggeringly large gift certificate for a whole lot of lab time.