You can easily judge the character of a man by how he treats those who can do nothing for him.

Malcolm Forbes (1919-1990) American billionaire

(Attributed)

Quoted in Earl Wilson, "Coco Offered Fatty Arbuckle Role," Hartford Courant (1972-08-06), the earliest reference found for Forbes.

A variant is found (without citation) in The Sayings of Chairman Malcolm (1978):

You can easily judge the character of others by how they treat those who can do nothing for them or to them.

Earlier versions of the sentiment can be found by Charles Spurgeon and Paul Eldridge.