Determining the CPU architecture of a static library (LIB) on Windows
Use dumpbin /headers
The machine type is almost the first line you'll get.
It will be 14c for x86 and 8664 for x64
n:>dumpbin lib642.lib /headers
Microsoft (R) COFF/PE Dumper Version
10.00.30319.01 Copyright (C) Microsoft Corporation. All rights reserved.Dump of file lib642.lib
File Type: LIBRARY
FILE HEADER VALUES 8664 machine (x64
Or
n:>dumpbin Lib32.lib /headers
Microsoft (R) COFF/PE Dumper Version
10.00.30319.01 Copyright (C) Microsoft Corporation. All rights reserved.Dump of file Lib32.lib
File Type: LIBRARY
FILE HEADER VALUES 14C machine (x86)