Material extrusion-based additive manufacturing (MEX) is an innovative method that has become widely used in the production of polymeric materials. With this method, final products have started to be produced beyond prototype production. However, inherent characteristics of the MEX process can lead to production-related discontinuities that, if undetected, may reduce part performance and cause safety risks or unexpected failures. Non-destructive Testing (NDT) methods, which have been applied in the industry for many years, are crucial in the detection of these discontinuities. To detect discontinuities in thermoplastic composites produced by MEX and to examine the application examples of these methods, samples containing 30 wt% glass fiber with polypropylene matrix were produced in the study. Discontinuities were artificially created in some samples, and the performance of non-destructive testing methods in detecting these discontinuities was measured. 0.1, 0.2, and 0.4 mm layer thicknesses were produced, and the effects of layer thicknesses on ultrasonic sound waves, thermal change rates monitored by thermal camera, and the detection of discontinuities in the sample were examined. In the ultrasonic inspection method, it was found that the increase in layer thickness improves the sound echo transmission and makes it difficult to detect discontinuities in samples with a layer thickness of 0.1 mm. Thermal imaging results revealed that layer thickness did not significantly affect the overall temperature distribution, but the discontinuity detection was more visible in the AL tape region with high thermal conductivity than in the PTFE tape region, and the rate of temperature change was slower in the AL tape region. Discontinuities were detected in NDT controls with ultrasonic inspection and thermal imaging, and it was shown that similar discontinuities can be detected with these techniques.