I am planning to install a master suite utilizing a two-car carport and a workshop already attached to my home. On the opposite of the only inside wall is a half of bath and in the workshop lives the hot water heater. I would like to know how to locate the existing pipes that I would use to tie in the new bathroom?

I take it your house is on a slab with no basement. Correct?
The most difficult will be the drainage. Since you don't mention any fixtures out in the workshop or carport the best place tp start is just down stream from the 1/2 bath toilet.
Wye off from the yoilet drain to pick up your bathroom group and install it like this, (ssee image). For the water I would locate the manifold in the wall, tie the hot and cold in and jump up in the attic and drop back down in a carport wall. Good luck, Tom

U will have to break up concrete floor to run new waste lines.

Under mine the wall with toilet on opposite side. This will locate waste line for existing toilet.

Then follow Tom's(speedball1) directions.

Chuck