If a dead short across a battery can damage the internals of some batteries then how do you know what the maximum current you can safely draw if it is not written on the battery ?

 

Connect a resistor of known value, measure voltage across the resistor, apply Ohm's Law. That is how I would do it.

 

There are rules of thumb based on battery chemistry and battery capacity.

This site has a lot of good info,
http://www.batteryuniversity.com/partone-16.htm