Here in the US genuine Glock magazines typically sell for $30-$35. This has been my experience in my area.
Packaging is always a clue. To duplicate Glock current packaging would be difficult and costly.
Pricing is almost always a great guide also.
Wholesales pricing on Glock mags is around $18 for most.
So if the cost is near or under dealer cost then it may be a copy.
I personally have never encountered anything that is a copy of a Glock product with actual Glock markings. Outside airsoft and products produced in other countries but not routinely sold here.