What might indicate that the piston is on the compression stroke?

When the piston is on the compression stroke, both the intake and exhaust valves are closed. This is crucial because, during the compression stroke, the piston moves upwards in the cylinder, compressing the air-fuel mixture within the combustion chamber. If either valve were open, the compressed mixture could escape, preventing the engine from building the necessary pressure for ignition.

With both valves closed, the air-fuel mixture is contained, allowing for an increase in temperature and pressure as the piston reaches the top of its stroke. This is essential for proper combustion in the subsequent power stroke, ensuring efficient engine operation. Thus, the state of the valves directly reflects the phase of the engine cycle, confirming the piston is indeed in the compression stage.