I work in schedules of 1500 lines or so and perform this analysis regularly. I have found the easiest way to do this is to view the Gannt chart on the right and have the timescale set to an interval level of daily. Then, with Show All Tasks, you can easily do a visual. If you have sequenced your tasks correctly as a waterfall, you can readily identify days where no work has been determined. I once found a two week gap that was not identified by reviewing the schedule any other way. Good luck!