A rule of thumb that makes sense to me is that communism generally differs from socialism in that it advocates one or more of the following:

- People actually living and working in communes.

- Abolition of private property (note: this is not the same as personal property).

- Extreme redistribution and/or equality in wealth/income.

I'm sure that this isn't a perfect heuristic; however, I think it's often useful for detecting when someone is trying to describe something as communism in order to make it look bad.