How can I measure a voltage between two conductive surfaces that are completely isolated?
Voltage is a measure of how much energy a unit charge would gain/loose if it moved through the electric field between two points.  Remember that 1V = 1 Joule per Coulomb.

But its just a measure of how much energy would be lost or gained.  That   potential exists weather or not you actually move any charges.  So voltage can certainly exist without current.

Objects can charge up to all kinds of voltages via static electricity.

Nothing is ever &quot;completely isolated&quot;.  Insulators including the towel, wood blocks, the bench, and even air don't have infinite resistance.  It may be very high but its not limitless.