You can get an electrical gauge to read the bottle pressure inside the car. I have an Auto Meter Pro Comp gauge but have never installed it because it requires a nitrous line to be run inside the cabin, and I didn't want that. The electrical gauge has a sensor in the engine bay (at the nitrous solenoids on the one I checked out) and then you only run the electrical into the cabin.

I have the SOS panel as well, but kept the front for a cupholder.