stalking their steam :D

Signs that it could be fake are....

Account is new. Only hours, days or weeks old, though I have seen some that are fake where the SG account has been around for 2 years.

Steam account is very low level. 10 and under usually, though I have seen some that were over level 10 create fake giveaways.

Very few games on their steam account. Though I have seen ones with fake giveaways with lots of games on their steam account.

Very low comments on their account. Though I have seen people with low amount of comments give away lots of games.

Some or all of the above with no comment in the giveaway area. Though I have seen some good giveaways with no comments.

Expensive/newer game being given away while no sale is on. Though some people will get it while its on sale and give it away later.

Expensive/newer game(s) being given away while its on the persons steam wishlist or at least not in the persons steam game list. This is usually a sign that they don't understand how this site works.

Expensive/newer game being given away with a private steam account. I don't think I have seen many with legit giveaways who have have their accounts set to private. (but I could be wrong on this one)

A game is being given away free somewhere on the net and person is giving away a game on the game list that is very close to it. For example people using Dead Island: Riptide to try to giveaway Dead Island: Epidemic beta invite.

People giving away a game, with a comment something like "I love this game" even though the game is not actually out or its not on their steam games list.

Very few giveaways entered. Though I have seen some actually give away games almost right away.

Joined x amount of days ago, last seen that same amount of days ago.

I think these are all the biggest signs on if a giveaway is fake. There are other signs too and you get used to spotting them.