The easiest way I found was to open Disk Utility and see which one was top of the list in there. For further confirmation, click on the volume there and look at the mount point displayed at the bottom - the startup volume is mounted at '/'. Other HDs will be mounted at /Volumes/drivename.
But why would you need to check that?
In my case, it's because both drives are bootable.
I was wondering why my SSD equipped iMac wasn't as fast as I expected. When I had a look it turned out that my old HDD had become the startup volume again - I'm guessing that a system update reset my startup volume for me.
It's simple to change back via System Preferences > Startup Disk and a reboot so not a major problem.
But why do I have two bootable drives?
Well... whilst it's possible to take an iMac apart and muck about with drives on the inside, it's an utter faff and frankly it seemed easier to get a thunderbolt / sata adapter, plug it in the back and use a cable tie to keep it neat. Thunderbolt is fast enough that an external SSD is just about as fast as an internal one.
I decided at the time to leave my internal drive as is just in case the change didn't work and never got round to changing it.