"How do you tell a communist? Well, it's someone who reads Marx and Lenin. And how do you tell an anti-communist? It's someone who understands Marx and Lenin." - Ronald Reagan
I don't think Mr. Reagan really meant that anyone who reads Marx and Lenin is a communist (I've read Marx, and I'm no communist), as the second part of the joke gives some important context for the first. Understanding Marx and Lenin usually requires reading them (as I have done), so we can take the first part of the quote to mean someone who reads Marx and Lenin without understanding them. But the second part can be taken literally, even precisely; which is why I find the joke funny. Those who understand Marx and Lenin are anti-communists.