How do I find the version of an installed product? (such as Reports, SQL*Net etc.)

One of the easiest ways is to run the product if you can. Running Reports designer will give you the Reports version, running SQL*Plus will give you its version etc.
Another easy way is to launch the Oracle Installer and look in the installed products on the right side. It will list all the products installed with their versions.
Another way is to look at the .rgs file. This file is located in $ORACLE_HOME/orainst
On UNIX, it will be called unix.rgs. On the PC, it will be called Windows.rgs.
This file will list all the products installed and their versions. Note though that
this file is not always correct. For example, patching Reports from 2.5.5.4 to 2.5.5.8 will not show up in this file as the new version. Running Reports Designer is the best way of getting the correct version.
Also, on UNIX there is an executable called 'inspdver'. It is located in $ORACLE_HOME/orainst.
Running it will produce a display of all the installed products. It just pulls the information from unix.rgs, so it may show the wrong information as well.