  An outstanding question concerning the underdoped cuprate concerns the true
nature of their Fermi surface which appears as a set of disconnected arcs.
Theoretical models have proposed two distinct possibilities: (1) each arc is
the observable part of a partially-hidden closed pocket, and (2) each arc is
open, truncated at its apparent ends. We show that measurements of the
variation of the interlayer resistance with the direction of a magnetic field
parallel to the layers can qualitatively distinguish closed pockets from open
arcs. This is possible because the field can be oriented such that all
electrons on arcs encounter a large Lorentz force and resulting
magnetoresistance whereas some electrons on pockets escape the effect by moving
parallel to the field.
