Once you have created and made use of a database link in Oracle,
you might want to keep an eye on the number of concurrent open database links
in your database so you can tune the open_links initialization parameter.

Read more here on limiting concurrent open links.

You will need to query v$dblink to see how many links are open in your session:

SQL> select in_transaction, count(*) from v$dblink
     group by in_transaction;

IN_   COUNT(*)
--- ----------
YES          1

Here are some interesting columns and descriptions of v$dblink:

db_link           Db link name
owner_id          Owner name
logged_on         Is the database link currently logged on?
protocol          Dblink's communications protocol
open_cursors      Are there any cursors open for the db link ?
in_transaction    Is the db link part of a transaction which
                  has not been commited or rolled back yet ?
update_sent       Was there an update on the db link ?

To gather information on all database links in your database, query dba_db_links.
You will need dba privileges to see this view, the alternatives are user_db_links and all_db_links.

A user who does not have dba privileges can query all_db_links to see
which db links are available to him.

SQL> select * from dba_db_links;

OWNER      DB_LINK     HOST       CREATED
---------- ----------  ---------- ----------
SCOTT      REMOTEDB    remotedb   23-feb-2008 23:59:05

Some interesting columns:

owner           User who owns the db link,
                will state 'PUBLIC' if it is a public database link.
db_link         Db link name.
username        The username that was specified if it was hardcoded
                during the create statement, null if not specified
                during the create statement.
host            The tnsnames alias specified during the create
                statement.
created         Date and time of link creation.

Recommended reading: