There is NO easy way to tell after all these years...
They were hard to tell apart just by looking when they were virgins way back when.
Now most have been into and the parts swapped up.
If it has a 5 bolt bellhousing or you KNOW FOR SURE the engine has never been into, then it can be done with some research.
Otherwise, you just gotta get in there and check all the individual parts.
If he got a 289 or 302, it doesn't really matter unless it was a 5 bolt block or late model with 50 oz balance that doesn't match his accessories. However, even then, it is a matter of "buyer beware".
Dave