Follow this terminal command:
more /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta.plist
If it isn’t up to date (as in today, or yesterday), go to Security under System Preferences, uncheck “Automatic updates, then immediately recheck the box. Run the command again, and hopefully you should be up-to-date.
Read more here: via How to force your Mac to update its malware definitions | Antivirus & Security | MacUser | Macworld.