Drop the script into any object, then click the face you want to know about. Its details show in your local chat:

[08:57:14] Kynno - Face Informations: Informations on click

Prim name: Kynno - Face Informations
Link number: 0
Face touched: 3
Color of face: <1, 1, 1>

Handy when you write Instructions reference, which need each prim's name and face numbers.