Is this a original waffen ss belt buckle?

[Larboard]

Without going through the whole SS buckle sticky thread, how can you tell cast from injection molded at a glance?

[Joda]

Without going through the whole SS buckle sticky thread, how can you tell cast from injection molded at a glance?

This is an example of zink injection molded buckle:
http://www.freiwillige-militaria.com/viewphoto.php?x=0 (if the link doesn't work, it is item 51542, buckles section)

Cast buckles are essentially aluminium buckles: they are similar to original ones just because "copied" on original examples, or even fantasy buckles built on the basis of a first model manufactured. The typical features of a cast buckle is the rough not-brighting white appearance aluminium, so a lower quality then wartime aluminium, and the typical little "bubbles" left by casting process. Little visible, but always present.
At last, the low quality of details, very "smooth" looking details.
Hope this helps..!