A method of determining the state of a printhead/cartridge in a thermal inkjet printer. An inkjet printhead undergoes a jetting operation in which a jetting frequency is selected and a corresponding steady state printhead temperature is known. The printhead is heated to the steady state temperature. Then the printhead is jetted with all nozzles for a predetermined period of time. Temperature samples from the printhead are obtained and the change in the printhead temperature for a short period of time is used to determine a slope in the temperature change. From the slope of printhead temperature changes, the ink flow rate through the printhead can be determined. The flow rate of ink through the printhead can be used to determine the various states of the printhead, including out of ink, clogged, deprimed, a taped printhead, etc.