How to Determine Target Architecture from Library or Executable Files

To determine the target architecture of a library file or executable file, simply type in
$ objdump -x <file> | grep architecture

For example, you could do
$ objdump -x a.out | grep architecture
architecture: i386:x86-64, flags 0x00000012:

So, we now know that it is for x64 architecture!

By the way, if you don't have objdump, you could get it for Debian
$ sudo apt-get install binutils
or for Mac OS X
$ brew install binutils && alias objdump=gobjdump