Omega geneve vintage solid 18k gold for woman - what reference?

You will have to have the watch opened to get the reference. These usually use a simple clamshell case.

Ladies watches have different trends than the gents watches which change slowly over time. Womens styles change at a faster rate with the seasons.

Gold watches are even more problematic due to import laws, Only the movement is sourced. The case made by somone local to the country of sale. There are a few well known casemakers. Finding definitive answers about such cases requires a bit of detective work. If one uses the 'site: ' keyword in google with omegaforums.net as the target there are a number of hits under the keywords 'ladies locally cased references.' with around 150 or so threads. It is a deep rabbit hole to explore.