They sell calibrated pressure gauges that give a fair indication. Not real accurate, bt least they tell you when it is getting near empty. LPG Gauge
But the better solution is to get a switchover valve, which provides an easy way to switch from tank #1 to #2 and includes a ""tank empty" indicator. Then when one tank goes empty, flip the switchover valve to the full one and take the empty to be refilled. That way you are never out of propane. LPG Switchover Valve
They even make an auto-switchover valve, but the drawback is that you may empty the second tank
Well you have a better ear than I have. I am never sure what a boink means as opposed to a bink. Besides you have to lift the fairing to boink or bink anyway so why not sneak a peek at the valve tell tale?