There are couple of ways to determine if RMAN database is registered with a catalog.

Using RMAN when you connect to RMAN catalog and try to run a RMAN command like “list backup” it will generate an error as shown below
calora4db01q:/home/oracle $ rman target / catalog rmancataloguser/rmancatalogpassword@catalogdb

Recovery Manager: Release 10.2.0.4.0 – Production on Sun Jul 17 08:33:42 2011

Copyright (c) 1982, 2007, Oracle. All rights reserved.

connected to target database: TESTDB (DBID=1023910334)
connected to recovery catalog database

RMAN> list backup;

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of list command at 07/17/2011 08:33:48
RMAN-06004: ORACLE error from recovery catalog database: RMAN-20001: target database not found in recovery catalog

Another way would be to connect to the catalog schema through SQL*Plus and check view RC_DATABASE
$ sqlplus rmancataloguser/rmancatalogpassword@catalogdb
SQL*Plus: Release 10.2.0.4.0 – Production on Sun Jul 17 08:45:58 2011

Copyright (c) 1982, 2007, Oracle. All Rights Reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 – 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select * from rc_database where name = ‘TESTDB’;

no rows selected