Look for:
NO numbers stamped near the valve hole
Welds on rear of the wheel joining the 2 parts of rim together
HOLDEN + date stamped inside the well of rim...

early gts wheels, (pre Sandman) will be stamped near valve hole, joined together with spot welds & have made in usa, or Canada stamped in the well of the wheel.

This is my observations & someone more knowledgeable may have differing or more info

Cheers
Max