If you win a gift, how do you know you have won?

A green bar will appear. Getting an email depends if the gifter chooses to give you the game through that mean or another.

you'll get a green banner in the front page saying "Congratulations, you have won a giveaway ..." along those lines

Thanks :) Now I know what to look out for.

A green banner will be present when you open steamgifts.

You get a green text on the main page similar to the one you get when your giveaway ends but telling you that you've won something instead.

FAQ

The Steam Gifts orchestra will march along your street, singing anthems about your epic win of Fortix.

the green text below the featured giveaway wont leave ur side till u either dun mark it received or later on mark it not received