Does anyone know from were I can identify the A/R for the exhaust housing for an HKS turbo?

 

You can always measure it. It'll be hard to determine the exact area of the inlet of the turbine housing, but you can get close. A/R is the ratio of inlet area to the radius of the turbine housing.

Some info here:
http://www.moddedmustangs.com/forums...ze-r-trim.html

 

i actually want to know if it is written anywhere on the housing

 

Probably not. Calculate it yourself, or try to find out exactly what turbo you have and look it up online.