To check whether a running instance is a RAC instance :

Multiple options here:

1. Check sqlplus banner (Applicable to Windows):

$ORACLE_HOME/bin/sqlplus / as sysdba

SQL*Plus: Release 11.1.0.6.0 - Production on Fri Dec 26 12:11:13 2008
Copyright (c) 1982, 2007, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, Real Application Clusters, OLAP and Data Mining options

A RAC instance will show "Real Application Clusters" option in the banner.

2. Check whether lmon background process exists for the instance

ps -ef| grep lmon | grep <ORACLE_SID>
oracle   627     1  0   Apr 15     ?  1:02 ora_lmon_racd

Only RAC instance has lmon background process.

3. Check cluster_database parameter

SQL> show parameter cluster_database

Output "true" means it's RAC instance but this is not reliable as a RAC instance may have cluster_database set to false during maintenance period.