Accelerating supermassive black holes, connected to cosmic strings, could contribute to structure formation and get captured by galaxies if their velocities are small. This would mean that the acceleration of these black holes is small, too. Such a slow acceleration has no significant effect on the shadow of such supermassive black holes. We also show that, for slowly accelerating black holes, the angular position of images in the gravitational lensing effects does not change significantly. We propose a method to observe the acceleration of these black holes through gravitational lensing. The method is based on the observation that differential time delays associated with the images are substantially different with respect to the case of nonaccelerating black holes.