There is provided a testing apparatus for analysing an aerosol generated by an aerosol-generating system comprising an aerosol-forming substrate and a mouthpiece. The testing apparatus comprises an airflow channel that extends from a first channel opening that is configured to receive a mouthpiece of an aerosol-generating system. The testing apparatus further comprises a sensing assembly comprising an emitter configured to emit electromagnetic radiation into the airflow channel and a receiver configured to receive at least some of the electromagnetic radiation from the airflow channel. The testing apparatus further comprises a pump assembly that, in a first configuration of the testing apparatus, is configured to draw air through the airflow channel in a first direction such that an aerosol generated by the aerosol-generating system in use is drawn from the mouthpiece received in the first channel opening and through the airflow channel; and a controller configured to determine at least one characteristic of the aerosol drawn through the airflow channel based on signals received from the sensing assembly. There is also provided a method of analysing an aerosol using the testing apparatus and a method detecting a defect in an aerosol-generating system.