Ask
Some are showers and some are growers. I consider myself a growing and even then I do not consider myself that big but I am 7 3/4" X 6 3/4". Yes I have been measured by one of those groups that measure. It was pretty interesting.
But I think the best way is still to ask. Sure, some people will lie about it and maybe some do not even know. If you think you have met someone who is lying, offer to 'help' them out and measure them. Tacky? Perhaps, but it will teach them a lesson.