I've seen several watches with solid gold cases and gold-filled cuvettes (the inner movement cover), so I would check the inside back of the caseback for any markings which would point to solid or filled gold.
A more interesting thing I noted was the regulator on the movement. That cut-out snail wheel I've only seen in Zenith watches before. On the other hand, I can't say if that means anything at all 
(and a vintage ad with the zenith form)
from 1909, elginwatches.org
l
A