first try installing something like Terminal Emulator. then open it.
it will look a lot like DOS (if you're that old) and you'll get a '$' symbol. type 'su' and hit return on the on-screen keyboard. if it pops up saying 'this app is trying to get super user access' with a 'grant' and 'deny' button, hit 'grant'. then type 'reboot recovery' and hit enter. if the device has a custom ROM it should boot into that custom recovery image. would show up as either ClockworkMod or TeamWin Recovery Project (TWRP). if it just loads the phone's stock factory backup software, it's not a custom ROM.
when you find that out, it's as simple as searching Google for the stock rom 'nandroid' image for your phone's model.
Rxpert, the OP is trying to determine if it already has a custom ROM on it already or not. it's possible to downgrade if it does, but highly unlikely if it was an OTA update.
Samsung Galaxy S3 (Stock, rooted TouchWiz)
Samsung Galaxy Tab 3 7.0 (non-rooted, stock)
Samsung Galaxy Tab 2 10.1 (obsolete, slow, rooted)
Samsung Galaxy Note 10.1 2014 Edition