The present disclosure relates to a sub-assembly of a drape-forming head (20), comprising, downstream, at least one member (23) for depositing the prepreg strip, at least one roller (30, 30′) for compacting the prepreg strip, and a device (10) for checking the play between the strips, consisting of a profilometer including a camera (13), a laser light source (11) projecting a line of light onto the surface to be checked, and a prism (16) arranged near the surface to be checked, in the axis of the camera (13), so as to send, to the camera (13), the image of the projected line of light that reaches the camera at a certain angle. The camera (13) and the light source (11) are placed side-by-side in an offset housing (18) arranged remotely from the compacting rollers (30, 30′), the prism (16) being connected to the housing (18) and to the camera (13) via an elongate casing (19) vertically arranged between the compacting rollers (30, 30′).