Many of those auto-cross over regulators have a simple pressure gauge on 'em with Red/green scales. If not any Major propane shop will have 'em.
These device are actually kind of a 2-stage regulator with selectable 1st stage
The "In use" tank.. if it has gas, delivers over 100 PSI to the main regulator input. and the gauge, which taps in here, shows GREEN
But when the main tank runs dry, pressure drops. Now the change over valve.. Which is a king of regulator, allows gas in from the 2nd tank BUT AT A MUCH LOWER PRESSURE the gauge shows red.
no reason the gauge can not be moved to a window by a good propane shop
I'd not want it INDOORS (Safety) but outside a window is fine
That is where it was on my Mother's Kitchen after all.