The flow behavior within fibrous preforms is of great interest for composites manufacturing due to the fact that filling defects have a significant negative impact on the structural health of the part and the production costs/time. When parts with relatively thick walls are manufactured (e.g. by RTM), the transverse flow and permeability become important. In this study a test-rig for the detection of transverse flows is developed and implemented. The central instrument of the rig is an ultrasonic transducer which allows continuous non-invasive flow front detection in a non-deflecting tool. Experimentation revealed the limitations of the system mainly related to wave attenuation but electromechanical enhancements in the rig raised these limits. The potential of the system to be used for transverse permeability calculations was demonstrated by experiments that involved ultrasound TOF measurements for woven glass and biaxial carbon NCF preforms. A model that correlates TOF with transverse flow front has been developed, implemented and visually confirmed.