If you select 2, 3 or 4 atoms some geometry information will be presented at the bottom of the screen:
Often you can also change the information displayed. The geometry of your molecule will be updated accordingly. This is not always possible: ring structures make it impossible since ADFinput does not know how to change other bonds and angles. In such a case you might temporarily remove a bond, fine-tune your geometry, and finally recreate the bond you removed. The planes of the plane angle are defined through (in order of selection) atoms 1, 2 & 3 and atoms 3, 4 & 5. The order of selection is important in all cases.
Alternatively, you can use the "Geometric Info" popup menu command (from the popup that you get by right cliking, or left-click-and-hold, on an atom, bond or in epty drawing space) to show distances and/or angles in the drawing area of the molecule. These are informative only, you can not use them to change the measurements.