Is there any indication when WiFi Assist is being used? If I have WiFi Assist turned on can I tell when/if it's being used?
There's actually no visible indication on the screen when it's being used (right now). I have noticed that when leaving home and getting on the road, the network switches from my wifi as I drive away onto the 4G a lot faster (without causing a drop in any streaming audio), but there is no indication on the screen that this is happening.

In short, it's a background feature.