The most reliable way I know to understand preemptive generation is to look for the debug log line each turn (you won’t see these in production unless you specifically enable debug logs, but you’ll see these in the console when running in dev mode).
If you see a using preemptive generation log it means that preemptive generation was used for that turn (you’ll also see a log if the generated result was thrown away). There is also a preemptive_lead_time that shows how much time was saved, in the attributes.
To configure preemptive generation, please refer to the docs here:
It is an attribute of turn handling.