  We propose a definition of externally measurable quantum work in driven
systems. Work is given as a quantum observable on a control device which is
forcing the system and can be determined without knowledge of the system
Hamiltonian $H_\mathcal{S}$. We argue that quantum work fluctuation theorems
which rely on the knowledge of $H_\mathcal{S}$ are of little practical
relevance, contrary to their classical counterparts. Using our framework, we
derive a fluctuation theorem which is operationally accessible and could in
principle be implemented in experiments to determine bounds on free energy
differences of unknown systems.
