You'd pretty much have to disassemble the motors to tell the difference. For the most part they're identical externally. Boss heads are closed chamber design with screw in rock arm pedestals. But a non Boss 351C could have them too. You'd have to look at all the casting numbers on the parts. There are some other minor differences between the two. A Boss 351 is one of the easiest Ford factory high performance engines to build a copy of from existing factory parts. The heads were just basically regular 351C closed chamber 4V heads with screw in studs for the rockers. The block and crank were nothing special. Nor were the rods. Standard 351C engines are pretty tough and can make a lot of power from the stock parts with just a cam, carb, intake, and a set of headers.