First, ask him a question: "Do you like McDonald?".
If the answer is "no", probably he is a leftist.
Then ask the second question: "Why?"
If the answer is "because it tastes like shit", then he's not leftist. He's a normal person. I too dislike McDonald.
If the answer is "because it's blulrbb… mega corporation... blurb… american imperialism... blurb... global capitalism… blurb"..., then... there you got your leftist.