do uvmap to cloth but always not to know what is front and back of cloth,

but when marking front and back so that can do texture map separately for front and back of cloth.

what is the way to mark front and back when do uv map?

progress as follow:

1:creat obj

2:do uvmap

3:render uvmap

4:not to know that how to mark front and back of cloth 

in uv map

Thanks