I think that most trigger_ and func_ entities are tied to a brush in the bsp file. Entities tied to a brush can't be spawned dynamically, only built into the map.
Find an entity in the entity list of the type you want and check it's model name. I believe an entity tied to the world will have a number for the model name (something like "*8" or "8*" instead of "models/path/modelname.mdl").