Obey the law.
As a concealed permit holder, you are only legally permitted to use deadly force if you believe your life, or the life of someone else, is in imminent danger. You are expected to exercise this option as a last resort, when all other options have been exhausted (e.g., escape, calling 911, etc.)
If you're dealing with an uncertain situation, ASSUME NOTHING. That scruffy-looking guy pointing a gun at a man in a suit could be an undercover cop making a legal arrest.