how to tell if a bettery is leaking?

hi all,

newbie question here...

I was wondering if there was a definitive way to tell if a battery
(nicad/ li-ion or whatever) was leaking. I have seen batteries leak
before (usually results in a white residue certainly in the case of
nicads) but is it ALWAYS obvious to the naked eye? Is there any way to
be sure a battery is intact? I was thinking of using litmus paper but
that really requires a solution of some sort to test...

In a similar vein, what causes batteries to leak? over charging?
disuse?

I have tried to research this but I have not uncovered a conclusive
test to ensure a battery (of any type) is correctly sealed.

Any information on this topic would be most gratefully received - I am
keen to understand exactly WHY a battery might leak.

gav