How can you build a metal finder?
The easist way to find nails and other steel is a magnet. Since this question is posted in 'Building and Carpentry / Technology' it begs the question, 'What kinds of metal are you interested in?' Yes, a magnet will help locate most nails and other ferrous metals (steel, iron), but not aluminum, copper, nor brass, all found in construction. There are commercial metal detectors available for purchase for this purpose, and a Google search for "How to build a metal detector?' produces many links to videos of inexpensive projects. All of these are likely to detect any metal at a shallow depth. Good luck. You will need an electronic oscillator whereby the frequency change can be detected as metal will shift the frequency of oscillation as it passes over or under the metal.