Zinc white detection

Let me consult with some fellow conservators about this. I did want to first sat that white the list of chemicals sold by Kremer is impressive, the all appear to be related to creating art materials (dyes, inks, laking pigments, grounds, etc), creating art (materials for printmaking, etc) and a large number for use by restorers/conservators.

We cannot think of anything that would work with wet oil paint. The following fluorochrome stain works wonderfully. The problem is that is used to stain cross-sections to be viewed under the action of UV light and the UV induced visible fluorescence shows the zinc contained paint layers. You would really need a ton of it if applying to films of dried oil paint on a macro scale and viewing with a hand-held UV light.
The stain is described here: https://en.wikipedia.org/wiki/TSQ
Sorry, beyond that you would turn to viewing dispersed pigment samples using a polarize light microscope,  from the up there is analysis with X-ray fluorescence analysis, and finally Scanning Electron Microscopy with Energy Dispersed Spectroscopy. These and other more rarefied instruments are likely way above any budget you would expend on this.
Perhaps there is an additional, less complicated method, I just have not thought of it.

​What about a spot test for zinc oxide?A Spot Test for Zinc White

Dilute sulphuric acid is added to a solution of potassium dichromate (1% by weight, 50ml) until the pH reaches approximately 1·6. Silver nitrate solution (0·125M, 10ml) is added slowly with vigorous stirring. After two hours, the supernatant liquid is decanted from the small amount of precipitate formed and the reagent is ready to use. No special storage precautions are necessary and the reagent has a ‘shelf life’ of at least a month; the loss of efficacy of the solution is indicated by the appearance of a dark red precipitate of silver chromate.
Antoni Palet Casas & Jaime de Andrés Llopis (2002) A Spot Test for Zinc White, Studies in Conservation, 47:4, 273-276.
To link to this article: https://doi.org/10.1179/sic.2002.47.4.273

​George, you are right and thanks. This procedure does require some toxic materials and others which would require a purchase from Sigma or other chemical supply house. Not everyone is comfortable or capable of pulling this off. For others, it would be a cake walk.