How to check if AC is low on refrigerant?

How to check if AC is low on refrigerant involves looking for signs and measuring pressures and temperatures with proper tools. Signs of low refrigerant include AC blowing warm or not cold enough air, evaporator coil freezing with ice on indoor coil or suction line, longer run times without reaching set temperature, higher electricity bills, hissing or bubbling noise indicating leak, and low subcooling for TXV system or high superheat for fixed orifice. To check properly, technician uses manifold gauges to measure suction and discharge pressures, low suction pressure indicates low charge, measures superheat and subcooling, low subcooling and high superheat suggest undercharge. Also check sight glass for bubbles if equipped. Look for oil stains around line set indicating leak. For homeowner without gauges, you can check air filter and airflow first, then feel suction line which is larger insulated line, should be cold and sweating but not frozen, if warm and dry may be low. Best to call licensed technician to check charge with gauges, leak detection, and add refrigerant after repairing leak, as handling refrigerant requires certification.