There are many switch systems that can be mounted to the frame that will detect when a conveyor is beyond the acceptable limits of alignment. A google of "Conveyor Belt Alignment Sensor" brings this within the first page.
Sensors will assist with major changes in alignment and sudden failures but they do not compare with the ability of a trained belt manager who walks the belts at least every shift.
While the travel rollers on a conveyor system can be set to correct for a minorly inconsistent alignment of the conveyor structure it adds additional wear to the bearings, rollers and belting. The additional wear will also reduce the efficiency of your conveyor if that is a concern to your operation. In all situations I recommend laser levels for the install of conveyor structure (even in semi-mobile installs) If this is a cross country type conveyor system that will deal with variable terrain crossing, vertical and horizontal curves or surface to elevated structure transitions using a knowledgeable surveyor is critical to alignment and the foundations should not be underestimated.
When major components on the conveyor are replaced or at the completion of the install the start up and training of the conveyor is important to insure that there isn't a failure due to premature wear. This site has a good set of basic knowledge about training and individual adjustment for travel rollers.