Two simple rules -

1. Don't act immediately (no matter how urgent matter seems to be).

2. When in doubt, check with someone you trust (and first reaction should be 'doubt').

Beyond that, any of following are worthy of being flagged as scam automatically -

1. Call/email from IRS or any other government agency

2. Easy money offers

3. (unfortunately) Anyone asking for help, specifically involving money, that too urgently

4. Anyone asking for password, SSN, financial record acess

Bottomline is that in online world, start point should be doubt followed by questions which help build your trust.