Burying the tubing 3" to 4" down should clear most bolts. If they
won't there's the expensive way - turn the system on and use some sort
of thermal imaging device to map out the pipes...

Or the cheap way - spray a little water on the floor, turn the
system up high, get down on your knees (I know, easy for some
people...) ;-) and mark where the floor dries out first with a
pencil. The marks will be right over the pipes.

I believe someone else here also used a third way:
Don't drill where the cat likes to nap. :^)
--Glenn Lyford