Comparing old and new values in a trigger

In the Below Example making check box to true if status field is changed to closed own 

// Check a checkbox only when an Opp is changed to Closed Won!

trigger Updatecheck on Opportunity (before update) {

  for (Opportunity opp : Trigger.new) {

    // Access the "old" record by its ID in Trigger.oldMap

    Opportunity oldOpp = Trigger.oldMap.get(opp.Id);

    // Trigger.new records are conveniently the "new" versions!

    Boolean oldOppIsWon = oldOpp.StageName.equals('Closed Won');

    Boolean newOppIsWon = opp.StageName.equals('Closed Won');

    

    // Check that the field was changed to the correct value

    if (!oldOppIsWon && newOppIsWon) {

      opp.I_am_Awesome__c = true;

    }

  }

}