When it comes to checking a heat exchanger for cracks, several methods can be utilized. One approach is to observe the flame when the blower starts. This can help identify any irregularities or changes in the flame pattern, which may indicate a cracked heat exchanger. Another option is to use a combustion analyzer to monitor the levels of oxygen (O2), carbon dioxide (CO2) and carbon monoxide (CO) when the blower is activated. Deviations from normal readings could suggest the presence of a crack. Infrared cameras can also be employed to detect cracks or damage in the heat exchanger visually. These high-resolution cameras capture images that can reveal any physical flaws. It's essential to rely on professional HVAC technicians with the expertise and tools to assess a heat exchanger's condition accurately. Regular inspections and maintenance can ensure your heating system's optimal performance and safety.