Good to see you here Prashant!
To me "good communication skill" means clarity of thought and the ability to explain your rationale succinctly.
When I am hiring I usually test for both written and verbal communication skills. You can easily gauge someone's verbal communication skills by having a conversation with them. Testing written communication skill can be a bit tricky, but not difficult. I make sure to converse a lot with any potential candidate over email so that I can gauge their writing skills. Some people are good at verbal communication, but fail to communicate effectively in written format. This is a big problem if you are building a remote team and spend a lot of time collaborating via Slack, emails etc. So, make sure you are chatting with the candidate, asking questions etc over email. In addition to this, I always pay special attention to the ReadMe in their assignment project. This is usually a good indicator of how good they are in conveying their thoughts in written format.
Hope this helps. 😃