How to know if you've won?

There'll be a giant green bar. You won't miss it.

When looking at the giveaway list a green bar will appear stating that you have won a giveaway.

Alright cool. Good to know.

you didn't win, in fact, you lost.

if you won, you will notice it

P.S: my first win was at about 660 entries

I come to your house and cook you dinner, hand you an envelope that tells you the coordinates of the steam key and you being full and ready voyage out into the night looking for it, plan to be out for at least 2 days, bring a weapon, its dangerous.

On very rare occasion you might not see a green bar. I've had it happen on two of my wins. You can check 'Your List of Wins' also.

In case you want to know what the green bar actually looks like: here you go.
You don't need to worry about missing it ;)

As said, you won't miss it. (Though random thought, I wish they'd change the "Your giveaway has ended give the gift" bar to something different, like blue. I get excited every time I see it for a fraction of a second before I remember).

But you probably won't win a giveaway just 100 entries in. It takes a while usually.

you get a green bar like I currently have

The best way to confirm is to check your wins page and see if there's any new entries.