I've heard many people say that, but they never quote Marx, so I can't look to see if they've misinterpreted him. I haven't read Capital (which is one of the reasons I hesitate to call myself a Marxist), but I've read a few other things, including Engels' short Socialism: Utopian and Scientific, and I haven't found any specific predictions for how socialism will come about.

Not saying you're wrong, mind you. I'm just saying I haven't seen the evidence to a claim that's often made. That may be my failure, of course.