Losing coolant in very hot conditions.

[robbo]

On my recent trip to the Spa Classic and the Black Forest, there were two occasions when some coolant was lost via the header tank overflow. Both times on hot sunny days stuck in traffic and limited to low speeds. I'm hoping the culprit is the radiator cap which has been ordered. However, although the fan operates within a few minutes on tick over from start up, as it should, is there a way of easily proving it would come on via the thermo switch in high heat conditions.
I know the manual suggests testing the resistance of the thermo switch  in a pan of water at various temperatures, but wondered if there's a simpler way.