You can change the opacity of non-active layers here:
You can also select Flash layers when it is selected which will make the contents blink white momentarily when you switch to a layer, but it only seems to work if you cycle layers via keyboard commands, not clicking.
Another trick that helps a lot is to simply ctrl+click on something with the pen selected. Holding Ctrl changes to the Move Tool and holding Ctrl with ther Move Tool enables the Auto-pick layer option. What this does is it will select the layer of the pixel you clicked on. It’s the feature I use the most in this program by far.