The test holes will give you an idea of what to expect. Sort of, if they are dug in the area where the dam will be built, and if they are dug as deep as the bottom of the core trench will be.
Moving dirt is like opening presents. You'll never know (for sure) what you will find until it's completely unwrapped.
As for FAQ's about test holes, not really.
As for clay, if none is found where the dam will be, check in other areas of the pond basin. The clay can always be moved from that area, brought to the dam area, and compacted.
The NRCS has a good "best practices" for pond building book. See if you can get a copy of it, or download it from their website. The guy that you want to dig your pond should know what a core trench is (and were it goes in the dam), how much water should be in the soil for proper compaction, how to compact soil correctly (typically a sheepsfoot roller or multi tired scraper loaded with dirt, NOT tracks on a dozer or excavator), how to compact dirt around the primary overflow pipe, how to properly size the primiary and secondary overflow (or have the resources to get those sizes), etc.
I can't stress enough that when you have your pond dug, do it right the first time. It'd be better to go slower and have everything planned out, with a good/great pond builder than to rush and have a dirt mover do the work. Leaky ponds can cost just as much to fix as it cost to dig it in the first place.
