  As quantum computers and simulators begin to produce results that cannot be
verified classically, it becomes imperative to develop a variety of tools to
detect and diagnose experimental errors on these devices. While state or
process tomography is a natural way to characterize sources of experimental
error, the intense measurement requirements make these strategies infeasible in
all but the smallest of quantum systems. In this work, we formulate signatures
of unitary evolution based on specific properties of periodically driven
quantum systems. The absence of these signatures indicates a break either in
the unitarity or periodicity condition on the evolution. We experimentally
detect incoherent error on a trapped-ion quantum computer using these
signatures. Our method is based on repeated measurements of a single
observable, making this a low-cost evaluation of error with measurement
requirements that scales according to the character of the dynamics, rather
than the system size.
