Many people use the "well-known scammers list", if you search GOG forums you should easily find it - it's maintained by good GOGers and most people on that list are indeed scammers who just collect codes and don't participate in the forums.
Also most of them have accounts from 2015, so esp. any summer account with low rep is usually a scammer. Not all of them, as there was a large influx of new people because of the Witcher 3, but you can usually easily tell by checking that list and if they're not on the list, just quickly put the person's name in the forum search box - if they have posts where they actually talk on the forums about stuff with others, then they're not scammers. :)
Btw some of them because of alt accounts have high rep (I think one had about 600), so when you give out keys via PMs, just double-check if they are not on the list. But in any case, the main thing is your good intent, so don't worry about it too much, just use the list and give keys by PMs, not dropping codes. Welcome to the forums btw! Hope you like it here! :)
-edit: Whoops, I just noticed you're an oldie, not a new member. Haha!
(RPGs are great!)
Post edited November 05, 2015 by Green_Hilltop