Hardness is water is made up of calcium and magnesium (natural minerals). The hardness cannot be seen with the naked eye, but can be detected through testing procedures.
I didn't know what hard water was, so I looked it up and here is an image I found. The image is of hard water that caused calcification on this faucet.
Well really that was a kinda stupid question but any way ;D hard water normally looks like ice or ice poles