Icons

Icons are used as a visual representation for actions, objects and intentions. Good icons are recognisable, attractive, noticeable, compact and understood by users. Icons can be categorised into 3 categories:
1) Resemblance - showing a physical object
2) Reference - represent a concept
3) Arbitrary - meaningful when it is a standard

To see if icons are understood by users, run tests by including icons as part of a user interface and by itself in isolation.