There is not really a specific way to tell if a PS4 has a certain firmware version. I guess the serial number could tell you what firmware it has, but many users would probably have to first make a reliable serial number database so you have a clearer understanding on what to look out for and what not to look out for...
a bit like the databases we have for the Wii U and N3DS XL at the moment.
I hear The Last of Us bundle comes with a 1.76 or lower firmware, but they're only rumors to me.