In a "normal" single disk installation,
disk0 represents the whole disk,
disk0s1 is the EFI system partition (user applications don't access this) and
disk0s2 is the OS X partition containing both operating system and user data. The following example is from a Macbook Pro running OS X 10.9.5 with a single 240GB SSD.
Code:
$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *240.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS MacSSD 239.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
If you have a second disk, it probably doesn't have an EFI system partition, so the first user partition on that drive might be
disk1s1.
If you select
About This Mac in the drop down menu from the Apple symbol, it should confirm the name of the startup disk (partition). Or use System Preferences -> Startup Disk.
If the drives are physically different (SSD and HDD) and different sizes, then the output of the diskutil command should be enough to tell which is which. In my case about I know that the 240GB SSD is installed in the default internal hard disk location.
Perhaps a copy and paste of your
diskutil list output would help.