How do index contours let you know if the land you are viewing is increasing or decreasing in elevation?

To show increasing elevation, the contours are drawn close together. If the elevation is decreasing, the contour lines are spaced out.

Index Contours are indicated by a thicker line compared to the others. Index Contours are labelled with specific elevations along it to give a better understanding of the scale of elevation.

The elevations on the Index Contour along with the legend of the map, that allows you read intermediate contour lines, gives you a clear perspective of increasing/decreasing elevation.