Hi,
I wouldn't recommend hitting cast iron with a hammer to see if it shatters as a safe method of identification. If you have a grinder you can use it to differentiate between steel and cast iron, or just about any other type of metal, by observing the sparks. Cast iron will have a very short spark stream - steel will have a noticably longer stream.
Check out this site for some diagrams of spark streams - hit the "next" link at bottom of page. http://www.tpub.com/steelworker1/5.htm
Do this in dim light so that you can easily see the difference in the carrier lines, forks, sprigs, etc. It will help if you have some material that you know is steel or cast iron to compare the sparks with. There is also information here on conducting a chisel test which will help if you don't have a grinder.
Have a blast,
Brian