To spy the cloven foot in any business; to discover some roguery or something bad in it: a saying that alludes to a piece of vulgar superstition, which is, that, let the Devil transform himself into what shape he will, he cannot hide his cloven foot.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cloven * To ChuckNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes