RSpec: how to test if a method was called?

In the new rspec expect syntax this would be:

expect(subject).to receive(:bar).with("an argument I want")

The below should work

describe "#foo"
  it "should call 'bar' with appropriate arguments" do
     subject.stub(:bar)
     subject.foo
     expect(subject).to have_received(:bar).with("Invalid number of arguments")
  end
end

Documentation: https://github.com/rspec/rspec-mocks#expecting-arguments

it "should call 'bar' with appropriate arguments" do
  expect(subject).to receive(:bar).with("an argument I want")
  subject.foo
end