The IBM PC/XT and AT shipped with what are now known as 83/84 key layout keyboards. The PC/XT keyboards are electronically incompatible with anything but an XT class machine, but the AT keyboards should be compatible with any AT keyboard controller -- even if the computer uses a PS/2 socket. You can tell them apart because XT keyboards have no indicator lights whereas AT keyboards have three status lights on the upper right corner. In 1986, IBM introduced a 101 key deluxe Model M keyboardfor the IBM AT which came to define their later IBM PS/2 line. The Model M and all newer keyboards are known as 101 key keyboards. Microsoft introduced the Windows key in 1995 for Windows 95, and keyboards with it are known as 104 key keyboards. That's the modern kind.
Generally, a keyboard with the function keys on the side and no directional keys will be from the mid or late 1980s. This was the original layout. IBM introduced the PS/2 connector in 1987, but many Taiwanese clone computers used the full AT DIN plug into the Pentium era and so many keyboards used that socket into the late 90s. Brand names such as Packard Bell and HP usually switched to the PS/2 connector for their 386s and late-era 286s, and a OEM keyboard with a full AT DIN plug is very old. AT/XT compatibility switches were a fad in the late 80s and early 90s and soon died out as XT class hardware got thrown out. Windows keys pretty much took over the market in 1995/1996, as Windows 95 was a huuuuge deal, so if a keyboard lacks a Windows key, it was probably made before then. Seems like rubber membrane domes edged out rubber dome and slider, foam and mechanical switch technologies that used individual keyswitches bolted to a PCB around this time.