Windows command to tell whether a .dll file is 32 bit or 64 bit?

DUMPBIN is included with Visual C++ and can provide this information with the /HEADERS switch.

Example output from a 32-bit image:

FILE HEADER VALUES
     14C machine (i386)
       6 number of sections
306F7A22 time date stamp Sun Oct 01 22:35:30 1995
       0 file pointer to symbol table
     1D1 number of symbols
      E0 size of optional header
     302 characteristics
            Executable
            32 bit word machine
            Debug information stripped