Don, Slabs are going to crack, How much depends on alot of different things .
1. how much fill is under the slab.
2. What kind of fill is under the slab.
The quantity and type of fill is not nearly as important as how well the fill was placed. Nearly any engineered fill can be made to be stable, but it's a matter if placing and compacting it correctly, and at the right moisture content.
As an aside, I've seen a case where a house was built on 20' of engineered fill, and when it settled 8" from one end of the den to the other, we determined that it wasn't the fill that was moving, but the loose native sands 20' to 30' below the fill. The native soil was stable until the homeowner started overwatering her lawn. She was watering the equivalent of 8" of rainfall per day -- our drill rig found saturated sand at least 30' down.
3. how much moisture is in the fill at the time of the pour.
Other than right at the surface, the fill moisture has very little to do with surface cracks in the slab. If the fill's bone dry on the surface, it can cause cracks as it sucks the water out of the bottom of the freshly-placed slab, but once you get an inch or two below the surface, the fill moisture doesn't play much of a role. Unless, of course, the fill was not properly placed, but then it's more of a compaction problem than a moisture problem.
4.how fast the slab dries.
This can account for hairline cracks running different directions in the surface of a slab, but generally not continuous cracks that run through the slab like Don described. You sound like you have experience with concrete, Donald, but for anyone who's interested, here's a pretty good overview of the different types of concrete cracks.
http://www.cement.org/tech/faq_cracking.asp
And I could go on and on but there is no exact science on what a slab will do. When the slab is poured the mason makes a guess where the joints should be and how many there should be. Sometimes they get lucky and sometime they don't. When the rebar steel or steel wire placed where it should be the crack will not be a problem.Tell your wife not to worry and enjoy the house.
I fully agree.
Hope this helps Donald Wood
On a lighter note, you could soak it in DNA and wait a few weeks
But where would you find a bucket big enough to put it in?