Method and apparatus to detect and characterize a defect in a fiber preform residing in a preform-receiving cavity of a mold or simulated mold of the type used for liquid composite molding (e.g., resin transfer molding) to make a composite component or structure. The defect may result from fiber preform misplacement in the cavity, accidental inclusions, preform density variations, and/or distortion of the preform. Transient gas or other fluid pressures at multiple locations of the cavity are measured during changing of fluid pressure in the cavity. The measured transient pressures, or data derived therefrom, are analyzed to determine membership of the fiber preform in one of two or more groups that include a standard defect-free group and a defective group, whereby the method determines if the fiber preform or its position in the cavity deviates from the standard defect-free group.