What is the test for determining whether a transaction or entity has a valid, non-tax purpose?

"[I]f a transaction or entity has no valid, non-tax purpose, nominally uses another person, or entity as a conduit through which to pass title, or brings about no real change in the economic relations of the taxpayers to the income in question, the Commissioner has the authority to find that the transaction or entity lacks economic substance and disregard it for tax purposes." Richardson v. C.I.R., 509 F.3d 736, 741 (6th Cir. 2007) (internal punctuation and citations omitted).