@aArczi Use ProjectileHitEvent/EntityDamageEntityEvent. If the 'damager' is a snowball and the 'shooter' of the snowball is a player, do whatever you want.
@aArczi If you're asking for code, we do not provide that here. If you want documentation: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/event/entity/EntityDamageByEntityEvent.html
@aArczi Here is the wiki page how to create events: http://wiki.bukkit.org/Event_API_Reference Simply register your listener class, add the event method, and add the lines that check if the 'damager' is a snowball and if the 'shooter' is a player. If you don't know how to get the shooter; cast the snowbal to a "Projectile" and use the "getShooter" method: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/entity/Projectile.html#getShooter()