Final answer:
Incomplete combustion is indicated by the production of carbon monoxide (CO) due to limited oxygen supply, distinguishing it from complete combustion, which only produces carbon dioxide (CO₂) and water (H₂O).
Step-by-step explanation:
An observation that would indicate there is incomplete combustion would be the presence of carbon monoxide (CO) as a by-product, rather than just carbon dioxide (CO₂) and water (H₂O). Typically, complete combustion of hydrocarbons in excess oxygen yields only CO₂ and H₂O as products. However, when the oxygen supply is limited, CO can be formed, which is a clear sign of incomplete combustion. This is an important consideration because CO is a dangerous, odorless gas that can lead to carbon monoxide poisoning, and it is also less efficient in terms of energy production.