A simple and accurate device monitoring dynamic focal shifts, occurring during propagation and pumping of a laser when the near-diffraction-limited laser beam is focused with a lens, is developed using a simple geometrical description of the laser beam. Shifts in focal position are normal between low power alignment shots and full-power shots in laser-plasma interaction experiments, and are attributed to the pumped rods acting as weak negative lenses, thermal stresses in the rods, or dynamic changes in refractive index. Masks with aperture spacing used in the monitor are described. Difficulties in alignment, monitoring operations, and measurements are discussed. It is inferred that much of the shot-by-shot variation found in experimental studies of laser fusion may be due to these focal shift variations.