Engine Swap Papers

Ok I have been asking around shops and all of them say the same thing, If I buy an engine from them I have to pay taxes to get papers for the swap. I'm guessing they don't sell stolen engines so if I were to ever get pulled over and they told me to pop the hood is there any way to tell if they engine is legit but without the papers? Is a legit engine without papers equal to a stolen one from a cops view how would he know wether it's legit or not if there aren't any papers?

well i doubt many cops know which engines comes stock nowawdays..but
if your civic is not new,you can easily say..i IT came LIKE THIS!
or say you bought it off a friend,aslong as the serial of the engine comes up to your name.