how do you tell if you win?

A green banner will appear on every page that tells you. You will know when it happens.

A green or blue banner will appear below the featured giveaway in the main page, and in every giveaway.

You won't miss it. It's the big green bar of happiness!

A Unicorn tells you

You had bad luck, look in the forums with your stats you can apply for some of those private groups for unlucky.Good luck :P.

the end of the rainbow will be at your front door and a leppercorn will give you the code for the game

you get a information by mail and when you log in to steamgifts, you´ll see a coloured banner on the top of the page.
you can´t miss it when you open your eyes

Steamgifts tells your washing machine which calls you to tell you that you won a giveaway! XD