There may be better ways, but a quick and dirty way is 1) save the file to a new name. 2) Open the Resource Name column and at the first task in your schedule assign a resource named “Resource”. 3) Fill down the entire schedule to the las task with this resource. 4) Now go to the Resource Usage view and set it to Days in the time scale. You can scroll through the complete timeframe of the schedule and see if there are any days when the resource named Resource is not assigned – those days have no tasks assigned.