alliedmodders/sourcemod — pull request #7 by Bara20 (2014-05-28 13:33:12+00:00)
Title: Add CS_DMG_HEADSHOT define

but i don't know if it works in other games.

( I'm using this already in my only headshot plugin - https://forums.alliedmods.net/showthread.php?t=234169 and works fine in CSS/CSGO )