How to recognize real vs. fake anti-virus warnings

How can you recognize real vs. fake anti-virus warnings? In the case of AVG Anti-Virus, if it detects a virus it will prompt you to “Heal”, “Move to Vault” or “Ignore” a virus threat (as shown below).

Common fake anti-virus programs prompt you to buy a license, or click for a “free” scan as shown by clicking “more…”