Methods and systems for determining the presence and/or rate of a flow of a fluid sample include transmitting light through the fluid sample are disclosed. The methods comprise, applying a series of thermal pulses to the fluid sample, the series comprises a time interval between each thermal pulse, detecting transmitted light using a light detector; and determining at least one of (a) whether or not the fluid is flowing and (b) a flow rate of the fluid, based on an intensity of the transmitted light corresponding to at least one time interval.