What is the reliability of smoke coming out of the exhaust as an indicator of a fire?

The presence of smoke coming out of the exhaust can indeed suggest the possibility of internal fire conditions within an aircraft, but it is not a definitive indication of a fire on its own. Smoke can arise from various sources, such as oil burning or component malfunctions, which may not necessarily point to an active fire. Thus, while smoke is a significant indicator that something is wrong and merits further investigation, it should lead the crew to assess additional data and indicators before concluding that a fire is present. This comprehensive approach ensures that any fire response is appropriate and timely, taking into account other diagnostic methods and onboard systems that monitor for fire conditions.