Cracked Head

I have long story to get through before I get to my question. I have a 2005 Kia Rio with around 71K miles. My timming belt broke about 3 weeks ago. I took it to a shop where they replaced the belt, rebuilt the head and replaced the pistons. When my husband went to pick it up he didnt get 2 blocks down the road and the car was smoking. The water pump had gone out so we replaced that too. I picked it up on a Thrusday, car was running fine. On Saturday the car started making a really loud squeal when I would hit the gas (would stop when I took my foot off the gas) and acted like it wanted to stall. The car never stalled but wouldnt go any faster than say 10 miles an hour. I wasnt far from home and didnt want to leave it so I costed it home. The check engine light finaly came on and when I pulled in my drive way there was smoke coming from under the hood, when I looked there was oil. I took it back to the shop where now he is telling me he is pretty sure it is a crack in the head. (He isnt done running test) and that will run me around 700 for the part. He said the crack more than likely happened when the belt broke. He said a crack in the head can easily be missed, and there is no way of knowing there is a crack until it starts leaking and making a big problem. So my question is this. When you rebuild the head, can a crack be missed? Isnt there some test to do to keep this from happening. Any help would be great.