Measuring state of charge of a non rechargeable CR123A lithium battery
Standard CR123A comes in variety of brands and chemistries. Their standard loaded voltage is 2.5 Volt at 700mA load current. This data can be easily obtained from internet. 

Now the easy way to tell how usable the battery is (not actually gauging left over charge) by loading it with a 3.3 ohm resistor while measuring its terminal voltage. If the voltage drops below 1.5Volt, you can assume the battery can be send for recycling.