small block 400 (and big block 454) WILL have a counterweighted harmonic balancer. Also have counterweighted flywheel but its not as easy to see that one.. dipstick location should be on the drivers side of the motor, unless its a newer block (forget the yrs but i think early 90's and up)
valve cover height is not an accurate way to tell (its not even a non-accurate way, its just no way to tell
exhaust manifolds, now that would be a tuffy unless you were and exhaust manifold expert/specialist and could identify many different types/casting #'s with the naked eye. either way if the motor was swapped out, they probably used the original manifolds, and chances are pretty good that over the years one or both may have been replaced.
as for the balancer, start the engine, let it idle and look up and the back side of the spinning balancer (i.e.crawl behind the front wheels and look forward to the balancer)... thats probably the easiest way if you are not sure what a counterweighted/nonCWed one looks like to begin with..
PS- are you wondering if its a factory 400? or do you know that its a factory 305? btw what year?