A liquid detecting system for an electronic cigarette includes a light source; a light detector; a liquid store including an internal cavity for holding liquid; and a light guide optically coupled to the light source and detector such that light emitted from the source propagates towards the detector along an optical path including an internal reflection within the light guide at a boundary with the cavity, such that light received by the detector along the optical path varies depending on contents of the cavity. Also provided are a liquid cartridge and an electronic cigarette including the system, a method for determining contents of the liquid cartridge and a computer readable medium including executable instructions which, upon execution by a device, cause the method to be performed.