For me, the communication skills are mainly around the following:
- Being able to convey complex topics in a way that is relevant to different audiences (technical and non-technical);
- Knowing when and how to give bad news about project progress / cost overruns / risks and issues in a way that allows constructive discussion;
- Being able to take the lead and facilitate discussion about how to resolve issues and problems, or how to develop an approach to allow solutions to be identified and developed. This requires the ability to elicit input from groups of subject matter experts who won't all be equally effective communicators, but who may all have something of value to add;
- Being able to stay on track when others are trying to pull the discussion into different - often irrelevant - areas, and to do so without upsetting people.
But your question was about evaluating the skills. This is a complex matter which can only really be done in hindsight. However, assuming that you are looking for methods to use in interviews, one option may be to ask the candidate for examples of when he or she had communicated the different types of message in the past: ask what exactly had to be communicated, how was it conveyed (verbal? in writing?), who were the audience, and what was the outcome. Add on "what would you do differently now?" and you have a reasonable set of questions... but you must still be aware that some people will always "talk a good game" while others may be less confident, but equally effective.
You might also ask about the normal means of communicating in a candidate's previous organisation, and ask for a description of the pluses and minuses of each. The candidate should be able to explain what works and what doesn't work for the different types of communication - so, for example, a few PowerPoint slides really isn't enough to communicate a complex technical design, but a regular weekly update on progress might be done using that medium. Equally, a weekly status report might possibly be done as a verbal update (depending on the circumstances), and should almost certainly not be a 40 page densely written report. If the candidate can explain this sort of thing, they have at least a grasp on what good communication looks like, and that's a major step in the right direction.