[klippy]

Turns out it checks if spawnflags & 8 is true for the "worldspawn" entity. That spawnflag is set if "worldspawn" entity's keyvalue "defaultctf" == 1, in CWorld::KeyValue. I don't know if there are any other cases where spawnflags may be OR'd with 8, but it's unlikely because this decision has to be made while the world is spawning (before any other entities have spawned I think).
Now I'm not quite sure if you can do operations on the "worldspawn", such as calling pev to get pev_spawnflags, you'll have to figure out the rest yourself. May as well hook KeyValue.