If you are genetically qualified, you could learn dowsing. If you want to do it electronically, there are several different approaches that work reasonably well, depending on distance and metal size. There are plenty of DIY versions of metal detectors available in the Internet. Be prepared for a huge variation in capability. I tend to agree with @Colin Mitchell concerning your ability to "make one much more powerful than the off the shelf type". This is a very mature market filled with hyperbole and unfulfilled promises.