Allah knows the fraud of the eyes, and all that the breasts conceal.
Remember, while you may be able to look at things which Allaah has forbidden without letting those around you know that you are looking at them, Allah will always know.
When you sit with your friends and a woman passes by and you steal a glance at her without them knowing, know that Allah knows full well what you are doing.
He also knows what you conceal inside your heart. You can fool the creation. You can hide your vices from the creation. However, nothing is hidden from Allaah.