How to test the maximum secondary current of a step down transformer?
I got a small step down transformer from my local market which has a
  total of 6 individually isolated winding in it (1 primary of 230V and
  5 different secondary wingding with 5 different voltage).


And


  As this from a local market it has no datasheet.


A transformer that drops AC dangerous voltages to much safer lower voltages is: -

A COMPONENT THAT IS RELIED UPON FOR SAFETY


So, if it doesn't have a data sheet and you bought it from a market I have to point out that you are risking your life using it. I would not use this device at all. Go buy one with a data sheet from a reputable dealer.