How to check whether a library is compiled to 32-bit or 64-bit code on AIX

Use the nm-utility:

"Displays information about symbols in object files, executable files, and object-file libraries."

In the following case, the Library file is 32-bit, since using the -X64 flag is throwing an exception:

nm -X64 /u01/oracle/product/ora11g_client32/mylib.so

0654-210 /u01/oracle/product/ora11g_client32/mylib.so is not valid in the current object file mode.
        Use the -X option to specify the desired object mode.

while using the -X32 flag will produce output similar to the following (abbreviated):

z00u070:ar3u>nm -X32 /u01/oracle/product/ora11g_client32/mylib.so

                     f           -
../../../../../../../src/bos/usr/ccs/lib/libm/m_tables.c f           -
zstcXAForget         U           -
zstcXAOpen           U           -
zstcXAPrepare        U           -
zstcXARecover        U           -
zstcXARollback       U           -
zstcXAStart          U           -

Note that you can use the OBJECT_MODE environment variable and instead execute

export OBJECT_MODE=64
nm /u01/oracle/product/ora11g_client32/mylib.so