A properly licensed PC should include either a Genuine Microsoft Label or a Certificate of Authenticity (COA) Label. See the following examples of labels to look for. If you see one of these labels without security features, look for an accompanying COA.
The color shift on the label is designed to change from one color to another when viewed from different angles, making it easy to determine if it is genuine.
Windows 8: magenta to green
Windows 8 Pro: blue to purple
Windows RT: turquoise to purple