I just built libpng on a 64-bit Windows machine using VS2008. It produces a libpng.lib file inside the \projects\visualc71\Win32_Lib_Release directory (Configuration used being "LIB Release"). 
I used dumpbin to inspect this LIB file:
C:\Temp\libpng-1.4.3>dumpbin projects\visualc71\Win32_LIB_Release\libpng.lib
Microsoft (R) COFF/PE Dumper Version 9.00.30729.01
Copyright (C) Microsoft Corporation.  All rights reserved.
Dump of file projects\visualc71\Win32_LIB_Release\libpng.lib
File Type: LIBRARY
  Summary
         8E4 .debug$S
         DF2 .drectve
        2BCD .rdata
       21165 .text
C:\Temp\libpng-1.4.3>
It does not however show the architecture of the LIB file. How do I find if a given LIB file is built for 32-bit or 64-bit architecture?
 
    