how to tell BSI on a nokia battery
 

Does the BSI have a symbol.

I see + ve - ve on most batteries.

How do I know which is the BSI if it has 3 pins or 4 pins when I only see + and - on a battery

BSI = Battery State Indicator

usually the pin of the middle is the BSI in most Nokia batteries , and if you aren't sure just find the phone's battery connector pinout in service manuals or on the net then you will find its battery bsi pin

if ther is only + and - then there is no bsi pin

-ve is the same as ground ?

It between them :D
In old batteries, on new - non-marked pin - BSI.
I not remember 4pin battery on Nokia BB5/Xgold/Lumia.