You don't, and most likely won't hit water. In most cases you will want to site the pond where you desire and then purchase and install a liner and pump.
Well you won't know unless you actually dig the pond yourself and there is no guarantee of hitting water. The water may have just all dried up due to weather conditions a while back.
You likely will not hit water by digging. If you want a pond in your yard, you will provide water, along with a plastic liner. Visit your local home improvement.