B5: How to identify an atom.
There are two ways to identify an atom. The easiest way is to simply place the mouse over the atom and the label will appear. You may need to zoom in to get this to work.

If you want to permanently label an atom, you can do so by following this series of commands with the menu:
Set picking¬Label
After you have completed the selection you can use the mouse to click on an atom to label it. Once you have finished labeling the atom(s), you probably want to restore the mouse action to just identifying the atom (the default mode) by the following series of commands: Set picking ¬ identify.

  What do the Labels Mean?
The labels that appear refer to the particular atom that was selected. Considering the following example:
[PHE]52.CB #399 2.45 -0.336 -0.308
   | It is contained in an Phe residue
          | Which is residue 52 in the protein
                 | The beta-carbon was selected
                       | It is atom number 399 in the coordinate file.
                             |x, y and z-coordinates of the atom.

If the structure contains more than one chain, then the chain to which the atom belongs is also shown.