Common signs of improper refrigerant charge in an HVAC system include increased energy consumption, reduced cooling or heating capacity, frozen coils, compressor overheating, and unusual system noises. Additionally, if the system is not maintaining the desired temperature, or if there are noticeable changes in system performance, it may indicate an improper refrigerant charge. Regular system monitoring and maintenance can help identify these issues early on, allowing for prompt correction and optimization of the refrigerant charge.