First thing to spot a bitcoin bonus scam is to trust your instinct, if you feel its seems to good to be true then it may be is so just go with your instinct. Some other points for bit bonuses site legit are:-
High return guarantee in short interval of time when you invest.
Gathering your personal or card information.
Very low fees charging by scammer.
Not a professional website or social media page, and no professional work done.
Unethical talks or behaviour