If it has no date code there usually is no way to tell.
On -most- USGI contract mags though, there is a date code stamped on the
side of the mag, somewhere. I have some OKAY industries USGI
mags, for instance, that show "7-91" stamped on the side.
The thing is, there are a whole crap-ton of USGI style mags out
there... I have a whole bunch that I bought that have no markings on them
at all. Your mileage will vary depending on what you run into out there.
-Mike