Lost81,
Can you use a sidewall as reference? In other words, do you want the face of the speaker perpendicular to the sidewall? And do you have a flat front face/plinth on the speaker? If so, take a framing square (one of those flat steel 24" x 18" L shaped thingies), and lay at the base of the speaker. Then measure from the sidewall to each end of the leg of the square which is sticking out away from the speaker. If the measurements are identical, then the face is perpendicular.
Or, use a laser pointer taped to the side of the cabinet and shoot to the back wall, or any surface in between. Again, the measure from the sidewall to the bulb should be the same as the sidewall to the laser dot.