• If dealing with a pushdown automaton, convert it into a CFG.
• Call a variable generating if it derives a string of terminals.
  1. Mark a variable X as "generating" if it has a production X -> w where w is a string of only terminals and/or variables previously marked "generating".
  2. Repeat the step above until no further variables get marked "generating".
  All variables not marked "generating" are non-generating (by a simple induction of the length of derivations).
• The language is empty if and only if the start symbol is non-generating.