Hi,
The problem you desribe is the classic chess piece identification task. Most sensory boards just detect the precence or abscence of a piece and infer its value from a know startiong position. eg the Novag Citrine which uses reed switches. The leading reliable chess piece identification system is the DGT board, which uses a patented process: see: http://www.chesscomputeruk.com/1999_-_DGT_Tasc_Piece_Recognition.pdf . The technology is Resonance coils of different types located in the pieces. A non trivial technology. The DGT board & pieces retail for around $1000. RFID is unlikely to work for the reasins given elsewhere. It is really an Identification technology. In theory you could use triangulation based on signal strength from 3 RFID readers, but I don't know of a way to force the readers to sequentially read all pieces in range. Plus the readers interfer with each other. There are "RFID Radar" devices out there that do this, but they use a large array of readers.