visually a cast crank will have sharp parting lines on the counterweights, forged cranks have thick parting lines. Probably a casting number would tell you as well. Most 383's were forged up until the later production years for this motor.
Try going to www.moparts.com/Tech (capital T in tech) and go to engine then big block, scroll down to crank Q's and ID. got pictures and easy to see difference. There was a cast crank in 71 on 2 barrel 383's other than that they were all forged and in 72 they were all 400's.