An object like a tomahawk would be very difficult to cast as there are tapers going multiple directions. With that being said, the easiest way to tell a casting is the parting line where the two halves of the mold met. If there was good cleanup and finish this detail may be obscured and not evident. Second thing is to look at the profile, most early casting require a fair degree of draft or taper if you will to allow the original mold form to easily be withdrawn from the mold. So look for a design that has taper in only one direction. Also with casting, small crosssectional areas sometimes do not turn out so well, so look for a beefier/meatier design. While these thoughts are not a know all, be all and there are always exceptions, these geberal thoughts can identify most castings.
thanks
Steve