In most of the scenario, We would need to identify that how many instances are there or How many instances installed in current server.
1. Login into SQL Server in which server you want to...
2. Copy & Paste the script given below and hit your 'F5' (or) Execute.
CREATE TABLE #Instances
EXEC('xp_cmdshell ''SQLCMD -L''')
DECLARE @ServerName VARCHAR(100)
SELECT @ServerName = CAST(SERVERPROPERTY('MachineName') AS VARCHAR)
SELECT LTRIM(InstanceName) [Instance Name] FROM #Instances WHERE LTRIM(InstanceName) LIKE @ServerName + '%'
DROP TABLE #Instances
3. Wow... Now you got the list of Instances installed on the current server.