I'm going to try that on my next dive where I drain my tank sufficiently.
So, to answer the OP's original question:
What are indications that a tank is getting low, other than the gauge, when you are underwater or do you take that last breath and that's it?
Dumpsterdiver touched on one indicator and that is a limp SPG hose. On your next dive note the stiffness of the high pressure hose and the amount of effort required to twist the SPG as the tank pressure changes. You should notice a marked difference between full and 500 psi.
It is not a reliable way to judge your tank pressure of course but it is the only other indicator of low pressure I can think of when the system includes a balanced first stage.