Yes, it's common sense which would save you from the scammers. Every day scammers popping with new scam techniques and only your brain could detect it at the beginning. There is no certain definition for the scam, you have to take a real-time decision. We know only a few basic scam methods like fake giveaway and Ponzi scheme.

For the free earning sites, you have to do your own diligence as much as possible. Remember, don't forget that nothing is free in the world, you are spending your time on the free site. If you have doubts about any site then you can share with the community to know more. Hope you will get a better solution always.