There are actually a couple of ways to do this. For versions prior to
8i, there's a utility in the $ORACLE_HOME/orainst called 'inspdver'.
This will list each product and version on your server. You can also
inspect the file $ORACLE_HOME/orainst/unix.rgs.

With Oracle 8i, you can run the Oracle Universal Installer, then click
Installed Products at the beginning.

Another way is to run the following SQL:

SELECT * FROM PRODUCT_COMPONENT_VERSION ;