EPTRoaches or infestors, they're harder because they can't automatically attack and give themselves away, but if your eyes are scanning between the minimap and the entrance to your nat, then they are really easy to see.
Observers, well, you sometimes get lucky. I have no idea how pros do it, but I'm guessing they're just used to seeing observers and recognizing the blur. Also, isn't that why zergs are starting to just put random spores around their tech buildings whenever they have money? It isn't extremely common, but it's a good idea.
But when you're looking at a deathball, it is pretty easy to see where the obsv is. It's just that you are looking directly at it and not worrying about injects at the same time. Usually, because for some reason the deathballs always come in the middle of a production cycle.
