test.rb

#!/usr/bin/env ruby

puts "ruby #{RUBY_VERSION}"

class A
  def a
    if respond_to?(:b)
      puts "I am responding to b."
    else
      puts "I am not responding to b."
    end

    begin
      b
    rescue NoMethodError
      puts "I can not call b."
    end
  end
end

class B < A

  protected

  def b
    puts "I can call b."
  end
end

B.new.a

# ruby 2.0.0
# I am not responding to b.
# I can call b.
#
# ruby 1.9.3
# I am responding to b.
# I can call b.