Look at the counterweights on the crank (the large chunks of metal inbetween the journals).
If it is cast, you will have a fairly thin ridge running along it as if you are looking at the edege of the two part mold it was cast in. If it is steel, you will have a wide (finger width) seam that runs along the counterweights. It will be as if a thick bead of weld was laid down, then ground smooth vs a more hap hazard 1/4" or less kind of pointed edged seam.