Word 2010 actually works, particularly with your example:

  1. Type: "I want less apples." 
  2. Click Review > Spelling & Grammar
  3. Grammar dialog appears:
       Number Agreement: "I want *less* apples"
       Suggestions: "fewer"
  4. Click Explain
  5. "Number Agreement

      A noun and the words that modify that noun must agree in number. "Many"
      and "few" modify plural nouns. "Much" and "less" modify nouns that cannot
      be counted or divided such as "much oil," "less happiness." In addition,
      the phrase "one of" must modify a plural noun."

Word 97 (and possibly earlier versions) also supported grammar checking for number agreement as evidenced in this review: http://www.ateg.org/monographs/haist.php