I have known about this for a long time. It is just one of those stories that get handed down. It came from my Mother who was not a Caldwell (obviously) and my Father just thought it funny.
My Mother said that a "Caldwell Characteristic" was the shape of their second big toe. A Caldwell second toe is of similar length to the big toe but narrow width and has a small toenail. This is accompanied by an extension of the skin joining the second to the third toe showing a marked "webbing" when compared to the line of the remaining toes.
As a child we had a few shoes and socks off with cousins and other relatives and she was invariably right.
When I met an old gentleman in Queensland 20 years ago who was my father's second cousin he was barefoot and the Caldwell toes were very obvious and a quick check with my feet showed the blood relationship.
I don't know whether this is just "our" Caldwell's or it is an ethnic thing or whether it is a common characteristic amongst people in general.
In the absence of general DNA data availability this might be a rough and ready test.
It would be interesting for the memory of my dear departed Mother if an informal survey could be made of interested Caldwell's to see if her theories were a nonesense or not.
In the interests of scientific research could you please remove shoe and sock (one foot will do) and check. Yes and no answers are needed to be fair.
We might need to organise a barefoot Caldwell festival.
(Yes I am serious - this is a research project of great significance)