3V lithium battery physics

Just load the battery with a resistor. In this case, around 1 kΩ would be OK, that would provide a nominal load current of 3 mA. Measure the voltage while the battery is loaded. If it is spent, it will become obvious. Most batteries will provide a voltage reading near nominal even when nearly completely spent, if left without a load for a while.
 
Thanks fellows but you are already over my head. I did do the preemptive thing a few years back and put in button battery holders in about twenty programmable units so I could swap out the battery easily and quickly. That procedure was plain awful, pulling out circuit boards so I could remove the deeply embedded soldered battery and then finding a proper place for the new battery holder.