foo_spec.rb

class Foo
  def hi; end

  def bar
    hi if @flag
    @flag = true
  end
end

describe 'some' do
  subject { Foo.new }

  context 'bar has not yet been received' do
    it 'does not receive hi' do
      subject.should_not_receive(:hi)
      subject.bar
    end
  end

  context 'bar has already been received once' do
    before do
      subject.bar
    end

    it 'receives hi' do
      subject.should_receive(:hi)
      subject.bar
    end
  end
end