In 30rd stuff there is no dating system.
You go for steel not polymer, and after research to know what subtlies exist from country to country, ideally stuff that comes from the former Eastern Bloc.
(ceased to exist prior to AWB, so obviously thier mags were made before AWB)
EG: East Germany
Drums are easier as they are more easily physically identified and some carry the mark of manufacturers/countries that were stopped from importing in 89 (beating the AWB by 5 years)
EG: the Chinese 'T36'
(has a triangle with 36 inside it stamped into the rear cover over the winding key, were blocked in the import ban and only produced up until around 91~92 regardless)
There are others, but that is one of the more common.
You're going to pay a premium for them though as they are drums, in limited supply, and highly desirable nation wide.
Side note: if you're planning on using AK mags in your Saiga you'll need to modify the mag release or they won't be able to lock in (done properly factory mag will still work fine) along with installing a bullet guide or the rounds won't feed into the chamber properly, if at all.