Pressure spikes that line up with water-heater recovery

Pressure spikes or an occasional discharge from a water heater’s relief valve seem like they could be related to thermal expansion, but I’m not sure what would distinguish that from a faulty valve or another pressure problem.

Does the timing help? If pressure rises mainly while the heater is recovering, would that point toward expansion in a closed system, while pressure changes when the heater is idle suggest looking more broadly? What’s a reliable way for a plumber to establish whether a pressure-reducing valve or check valve is making the system closed, and to verify that an expansion tank is properly sized and charged before recommending parts?