A method for checking a flexible line, the flexible line including at least one layer of armors (24, 25) surrounded by an external sheath (30), the external sheath (30) delimiting an internal space (33) receiving the layer of armors (24, 25) and including at least one medium (M) at the interface between the external sheath (30) and the internal space (33). The method includes sending an ultrasonic signal on a region to be checked of the external sheath (30), and receiving the reflected signal at the interface between the region to be checked of the external sheath (30) and the internal space (33) facing the region to be checked of the external sheath (30); and analyzing the polarity of the reflected signal and determining, according to the analyzed polarity, at least the nature of the medium (M) at the interface.