A test apparatus checks containers (13) of plastic and produced using the blow-moulding, filling and sealing methods. The containers are filled with fluid that can contain particulate contamination deposited on the container wall when the container (13) is still and floating freely in the fluid when the container (13) is moving and/or changing position owing to the movement. The contamination can be detected by a sensor (37). By a vibration device (23), the container (13) can be oscillated at a prespecifiable excitation frequency such that the particulate contamination (47) in the fluid can be detected.