The accurate detection and quantification of carbon dioxide (CO₂) in gas mixtures is critical across various industrial, environmental, and biomedical applications. This study presents a comprehensive evaluation of different testing methodologies for CO₂ measurement, including non-dispersive infrared (NDIR) sensors, gas chromatography (GC), and chemical absorption techniques. Each method is analyzed for sensitivity, accuracy, response time, and suitability under different operational conditions. Experimental testing was carried out on synthetic and real gas samples, simulating environments such as biogas, industrial exhaust, and ambient air. Results indicate that NDIR offers high-speed detection suitable for real-time monitoring, whereas gas chromatography provides superior accuracy for trace level analysis. This paper further discusses calibration standards, interference from other gases, and cost-effectiveness of each technique. The findings aim to guide the selection of appropriate CO₂ testing methods based on application-specific requirements.