Sometimes, carbon can have the appearance of metal particales. if you still have the oil, strain it through a rag and save the particales. Place the material on a flat surface and strike it with a hammer, if it shatters then its carbon, if it stays the same or changes shape then its metal.
The best thing to do is get a magnetic drain plug and run it for a month and then drain it. If theres material on the plug then its steel, or if its aluminum it will be in the oil. Steel is acceptible in certain amounts, but if its accompanied by aluminum, then its usually a sign of warped pistion rings. But in anycase, what are you gonna do? tear down your engine and do an overhaul? not worth it.