How to detect low battery of a lead-acid battery?
Fortunately, there is. You can find diagrams of the discharging process of a battery type, search for "lead-acid discharge graph" or so. This returns graphs like this one:



In the datasheet of your battery you can find the exact graph for your battery. 

Say you want a circuit to detect 80% discharged, just check in the graph, the voltage of a lead acid battery with 80% discharged is about 1.9V. 

You can check this voltage with a multimeter, or use a comparator circuit or a microcontroller with ADC to make a dedicated device for this.