Hi Norman,
The first place I would look to determine age, or if the possibility exists if it is old, would be age wear underneath. That is not always a good indicator though. If the piece is an older model that is being reproduced, then the quality of the mold work and finish quality of the glass can also be an indicator of age. These pieces can be tough, especially if you are not familiar with the many early patterns being reproduced, and the new patterns being done today..
I looked at a piece of Malachite glass yesterday at an auction preview that is an Artur Pleva designed piece produced by Curt Schlevogt in the 30's. It is an ashtray with three horses. It is also produced currently by Desna. The mold quality, detail and finish work were exceptional, and the piece had lots of age wear underneath. I also looked at the provenance, of being part of the estate of an advanced collector that had owned it for quite some time according to the family (but we all know how those stories can be wrong). I also evaluated the quality of the other items he had collected. All of this combined brought me to the conclusion it is an original piece.... But there is no real guarantee of that, other than my trust in my experience.
Craig