GIII Tachometer

In the older car, you could simply listen, and there's a bit of vibration when the car is stopped. I don't know about the new one yet.

However, don't worry about it. Most of the fuel economy comes from the differences in the engine compared to a traditional engine design, and that it's so much smaller. The electric drivetrain components permit such a small engine to be used, filling in where the car needs torque at the low end, and peak power for overtaking and hill-climbing. Trying to keep the engine off and using electric driving only tends to use more fuel to recharge the battery, over the entire cycle. You get good instantaneous fuel economy but poor averages.

You can put the MID into the 'Energy' display (press INFO on the steering wheel to switch the display, if I recall) which shows where energy is flowing. Some people have said they find the red arrows hard to see. The engine can be running without an arrow from engine to wheels, though, if it's still warming up.

If you really want to know, somewhere on these forums someone (Ken?) posted a mod to illuminate an LED when the engine's running. Alternatively, a ScanGauge will show the engine RPM.