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

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.