Active Data Guard Enabled / Disabled

How to Check Active Data Guard is  Enabled  or Not 

SQL> select  'Using Active Data Guard' ADG   from v$managed_standby m,v$database d where m.process like 'MRP%'  ;
        ADG
-----------------------
Using Active Data Guard

or from standby database

SQL> select open_mode,controlfile_type from v$database;

OPEN_MODE              CONTROLFILE
--------------------   ------------------
READ ONLY WITH APPLY   STANDBY

Conversion from Physical standby to Active Data Guard standby
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Real-time query
===============

A physical standby database can be opened for read-only access and used to offload queries from a primary database. In addition, if a license for the Oracle Active Data Guard option has been purchased, a physical standby database can also be open while Redo Apply is active. This capability is known as the real-time query feature.

Redo Apply must be stopped before any physical standby database instance is opened.

If one or more physical standby instances are open, those instances must be stopped or restarted in a mounted state before starting Redo Apply

SQL> SELECT open_mode FROM V$DATABASE;

OPEN_MODE
--------------------
MOUNTED

Stop Apply Services

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

Database altered.

SQL> SHUT IMMEDIATE

SQL> START NOMOUNT

SQL> ALTER DATABASE MOUNT STANDBY DATABASE;

Database altered.

SQL> ALTER DATABASE OPEN;

Database altered.

SQL>  SELECT open_mode FROM V$DATABASE;

OPEN_MODE
--------------------
READ ONLY

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE disconnect;

Database altered.

SQL> SELECT open_mode FROM V$DATABASE;

OPEN_MODE
--------------------
READ ONLY WITH APPLY

It enables us to have a physical standby read only open, while redo apply is still done in the background.
The Active Data Guard  contains the following features
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Physical Standby with Real-time Query
Fast Incremental Backup on Physical Standby
Automatic Block Repair