Get spawner

If I use this to spawn entity.
[lua]
local Category = “Strider”

local NPC = { Name = “Strider”,
Class = “npc_strider”,
OnFloor = true,
TotalSpawnFlags = 0,
Offset = 2,
Category = Category }

list.Set( “NPC”, “npc_strider”, NPC )
[/lua]

How do I get afterwards who spawned it?