The new release of Hybrid Assistant, for selected car models (Yaris, Auris, Prius3), will advise you about the engine stages.
At car power-on, a "Play" icon will appear on the engine icon if the engine needs to be warmed up and will start in a few moments.
You can prevent it by switching to N or selecting EV mode.
When the right conditions for an idle check are met, a "Pause" icon will show up.
If you begin to coast, the engine will perform the idle check and will switch to stage S4.