Communication skills are a combination of the hard skills of verbal or written communication combined with the soft skills of judgment and knowing how/what/when to communicate.
The former can be judged using assessments such as those used by many recruiting agencies but the latter is much more difficult to assess and might only be gauged by looking at the issue logs from past project the individual has managed to understand what percentage of issues related to poor communications on the PMs part.
I concur with Kiron.
I also think that you can evaluate someone's communication skills by looking at their ability to take the lead and facilitate discussion about how to resolve issues and problems. Saving Changes...