How can you tell if a site is scam or not?

It's important to know if a site is a scam or not from the beginning in order not to spend a lot of time and effort for nothing....

These are some of strong signs that tell us to avoid such sites: 

•  If a site pays users 0.10 per click or more  which is unreasonable.
•  Users can't make a request of payout until they invest some money. It’s a strong sign.
• If there are a lot of ads to click , this means that members have to use their cash balances into advertising because they can’t cash them out.  
•  No fourm on the site or the admin dose not respond to members support ticket.