How to know if someone is SCAMMING!

Spread what, exactly?
To not implicitly trust complete strangers with your belongings?
To ensure their steam profile is actually their own?

I hate to be blunt here, but this is really just common sense.

It happens a lot. It really sucks and I feel bad for you. But I have seen many many topics about this posted here in discussions and they don't seem to make any difference. Report the scammer on steamrep and hope the next person checks there first. Probably also best to tell the user with good rep that they have an impersonator.

Sorry to hear this, but it's nothing new. I could post a few guidelines about trading, but others will probably do it instead of me. The best you can do is to install Enhanced Steam add-on, because it shows you if the person has SteamGifts profile, and SteamRep as well. Scammers often link accounts with high rep, but this should be your first warning. The second warning should be, that the guy's Steam profile is private or friends only. You can check this by checking their account while logged in or off. It became very common, that they set their profile not private, but friends only. You can see their real level and profile, because they added you, but if you log out of Steam you can check if it's public or not.
You made 2 things right, which is unbelieveable. First: Although they linked you an innocent person's SteamGifts account, you didn't left a false negative rep there (did you?). Second: You are new to SteamGifts (6 months old, but no threads or posts yet, besides this one), but you didn't post their real Steam profile link, which is against the rules BTW. So you did these well. If the games were gifts you may be able to get them back by asking Steam Support, but that's highly unlikely. If they weren't gifts, then you have no chance, you had to learn the dangers of trading the hard way.

Be sure to install Enhanced Steam and I wish that your next trade will be better :)

Yup, I feel for new SG users. I've been through that, gladly I had my shared amount of other people's experiences prior to that, so I only lost a crappy game which was worthless anyway. :D

Somebody adds you and claims to be someone else?
Click the link to the profile he provides and follow it back to their steam profile.
If you end up on a different profile as the one who speaks to you then you found a scammer.

There is nothing more to it except common sense.

It's literally impossible to "know" for sure if someone is scamming, but it sounds like you fell for an impostor and that is easy to find out if its true. Click anyone's name here, and then go to their Steam profile. Read what you're looking at, check their Steam Rep. Install Enhanced Steam (Google it) and it will put links on people's profiles when viewed in Chrome to SteamRep and to here.

Its always possible, whether you're dealing with someone with a high amount of rep or otherwise but typically someone with higher reputation is far more unlikely to be scamming. You're at a greater risk because you're new (at least on this site) to trading and if you're going to trade outside the Steam trading window, it makes it a lot tougher to ensure your safety. Obviously, its easier to tell you're being scammed if you're dealing with a complete idiot, but the ones with brains are the ones that can get you and even an idiot can pull off a Paypal scam b/c one side always has to go first.

What I'd suggest you do is only trade with people with relatively high amount of reputation. It's still no guarantee that you wont be scammed if someone has to go first. Build up your reputation and then you'll be in a position to demand others go first.

I've mad 2 trades in here so far and they all went well despite going first.
1-Never trade with anyone unless he/she comments on your topic and lets you know that he/she'll add you
2-Check his reps on SG and make sure to visit 2 or 3 user profiles who've +rep him in order to make sure they're not fake accounts.
3-Next step is to visit his/her profile in "steamrep" to make sure the profile is clean.
4-If you don't feel safe then don't trade.
One last thing..... If it seems too good to be true,it probably is