If the site was properly prepared and the pour was on undisturbed or properly compacted soil then chances you'll have a problem are small. Using fibers in the pour minimizes the chances of cracks forming. Given you have an experienced contractor I wouldn't be too concerned but I would pay close attention to what happens for the next several weeks. After that, if nothing shows, you should be fine. Preparing the site is much more important for preventing cracks than the time of the cure since this pour is entirely supported by the ground.
ok. what should I look for as signs of trouble? hairline cracks? cracks in certain locations? size of cracks?