Is it possible to tell if someone is intelligent if they have poor communication skills?

The quality of someone’s decision making is a reasonable marker of intelligence. Look at the choices they make, and where they are in life in terms of relationships, earning, occupation. Don’t neglect emotional intelligence. Are they able to stay calm in stressful situations? Are they able to manage the ups and downs of daily life on the emotional level? All these play a factor in one’s overall intelligence.