This can be done by either 2 methods:

Method 1:

Go to $ORACLE_HOME/bin

then execute the command:

$ file oracle

oracle: 64-bit XCOFF executable or object module not stripped

Method 2:-

Login to sqlplus,

use the following query,

SQL> select metadata from sys.kopm$ ;

Search in the output string if it has B023 string then the database is 32 bit else if it is B047 then it is 64 bit