First thing's first - use a hygrometer to see if the humidity is decreasing. If you don't have a hygrometer you can always do the dollar test to see if the humidor has a good seal. Basically, close the humidor onto a dollar bill and pull it. If you can pull the humidor without the dollar coming out, then it has a good seal. Last resort is to just check your cigars every few days to see if they got any dryer, but I really don't recommend that.