Enlarging a shower stall?

We're undertaking a big remodel of our Eichler master bathroom, and this is probably the first of many posts to make sure we're not repeating anyone else's mistakes.

You can read more about the house and our design at http://paeichler.blogspot.com.

Today's question is about creating a larger shower without introducing a high threshold or curb around the perimeter that you have to step over. The original 3x3 square shower stall was sunk down an inch or two into the concrete slab when they poured it, and we're thinking about expanding that "footprint" for another 3-4 feet to form a rectangle with the required slope for water back to the original drain.

Obviously that would require digging out some concrete. So my question is how to determine if there is radiant heating pipes in that part of the slab?

Funny related story: previous owners had put a down-draft Jenn-air range in the kitchen island and routed the air vent down through the slab and back up the nearest wall to the roof. The story I heard was that they were digging out the required channel using spoons to avoid hitting the radiant pipes. Don't know how true that is, but we obviously need to take a slightly different approach since we need to create a gradual slope, not a channel.