I am indebted to my friend Kieran Wan for the acute observation that there is more to the size of men’s umbrellas than meets the eye. Men who have girlfriends have small umbrellas; men who are still hunting have large umbrellas.
The reasoning behind this is that men with girlfriends relish physical proximity. Having a small umbrella means you have to huddle together to avoid getting wet. This argues strongly for putting an arm round the lady, which is what you want to do anyway. Unattached men, on the other hand, wish to be in a position to offer generous and unthreatening umbrella accommodation to ladies they have not met but are interested in. So they have very large umbrellas of the kind usually found on golf courses and parachutists.
This leaves us with the question of married men’s umbrellas, and a possibly useful guide for suspicious wives. Happily married men have collapsibles which they can hide in their briefcases. If he switches to a large umbrella he is hunting. And if he moves on to a smaller umbrella the hunt has been successful.
I am not sure if this system works, but it is probably as reliable as looking for lipstick on his collar, and less intrusive.
tim's experiment 
Thanks for the tip, Tim! Very perceptive! Anything you may say about the size and number and types of cars? We bought a 7-seater since the birth of Preston but Andrew insists on keeping his two-door race-car-look-alike despite the economic imperative to cut down on spending. Anything I should worry about? ;P