There are 3 relatively simple ways to check which release version of Oracle you are running.

a) Determine the version or Oracle DB running on the server by looking at the startup sequence in the Alert log.

This method can be performed without making a connection to the database.

By default the alert log is written into C:\oracle\diag\rdbms\protect\protect\trace\alert_protect.log

The name of the file consists of alert_<SID>.log. The default SID of a DLP database is "protect". 

If the file is not present in the specified directory then it's location can be determined by following the article linked below:

Where is the Oracle alert_protect.log? (broadcom.com)

Open up the the alert_protect.log in a text editor of preference and search by term "Oracle Database". The next line to the search result will contain the exact RU (Release Update) of the Oracle Database running on the host.

Oracle Database 19c Standard Edition 2 Release 19.0.0.0.0 - Production
Version 19.16.0.0.0.

b) Determine the version by connecting to the database with SQLPlus. This will require connecting to the database.

1. SQLPlus should be natively available on servers hosting the database or with the Oracle Client installed. The version of the database will be displayed once the connection is established. In this example protect user and SID will be used since those are default for a DLP database. To connect to the database with SQLPlus execute the following in command prompt:

sqlplus protect@protect

2. Enter the password when prompted.

3. Once the connection is established SQLPlus will return the version of the database in the command prompt:

Connected to:
Oracle Database 19c Standard Edition 2 Release 19.0.0.0.0 - Production
Version 19.16.0.0.0

4. Close the connection by running the following command:

quit;

c) Obtain the database version by running a query. This method requires connecting to the database.

The Database version is stored in a system performance views named V$VERSION and V$INSTANCE which are by default present in any Oracle DB installation.
To retrieve the version information from the database:

1. Open up command prompt as administrator on the Oracle host, or another with the Oracle Client installed.

2. Login to the database with SQLPlus, or other management tool that may be installed in your environment. SQLPlus should be natively available on servers hosting the database or with the Oracle Client installed. In this example protect user and SID will be used since those are default for a DLP database. Execute the following in the command prompt:

sqlplus protect@protect

3. Once connection is established execute the following query:

select version_full from v$instance;

The result of the query will show the full version of the Oracle Database with the latest Release Update installed. An example result will look like following:

VERSION_FULL
-----------------
19.16.0.0.0

Alternatively run the following:

select banner_full from v$version;

The result will look like the example provided below:

BANNER_FULL
--------------------------------------------------------------------------------
Oracle Database 19c Standard Edition 2 Release 19.0.0.0.0 - Production
Version 19.16.0.0.0

4. Close the connection by running the following command:

quit;