Here is a nice tool if you are not aware of. On the installation screen of SQL Server, under Tools section, you will find “Installed SQL Server features discovery report”. See below image.
You do not need to install anything. Just click on that and the output is going to be self-explanatory. Basically, this reports lists out all the SQL Server 2000, 2005, 2008, 2008 R2 & 2012 instances on your box with edition & version information. For example, I have many instances installed on my box and this report gave be a quick info on that.