Any advice on identifying fake giveaways?

Some are quite obvious, their profiles were made an hour ago, and their steam accounts have nothing on them. However I've been looking around on the forums a lot and can't help but notice many people are being scammed of their giveaway victories. It's kind of sad, because the profiles of the people giving away the games seem quite active. They have hundreds of screenshots, many hours played in multiple games, achievements, friends, comments etc.

So my question is, does anyone have any useful advice to help relative newcomers such as myself differentiate between the legit giveaways, and the fake ones?

Thanks!