I have a post tension slab, I want to convert my laundry room into a second bathroom, move the laundry room into the garage where there is access to a sewer line. Can I break open a small section carefully to find the cable then undermine to new location to repipe the ABS?

I myself wouldn't go breaking holes in the slab to find cable locations. There are other methods to finding them. Is the home fairly new ? Do you have access to the plans ? If not you could try a metal finder and trace the floor with chalk to give you a grid reading.

Its not a good idea to disturb a post tention slab.

you could try a metal finder and trace the floor with chalk to give you a grid reading.
I would , for sure, want to know exactly where the cables are before I made any cuts. Dads suggestion was bang on. Good luck, Tom