In laser-ion acceleration experiments, the interaction of the rising flank of a high power laser pulse with the target can cause pre-ionization and subsequent target pre-expansion long before the arrival of the main laser peak. Exact knowledge of this target pre-expansion is required in order to understand the laser-plasma acceleration mechanisms with the help of numerical simulations. For dielectric targets, the starting point of target pre-expansion is characterized by the point in time at which the target undergoes laser-induced breakdown (LIB). In this work, we present a method to determine the time of LIB in sub-micron-thick Formvar targets during interaction with a specific high-power laser pulse. The required pulse-duration-dependent LIB threshold of Formvar is measured in a dedicated experiment. A comparison of LIB threshold to previously published data facilitates a generalization to other wide-band-gap dielectric targets for laser-ion acceleration.