That would be the problem. Even stellar events like novas and pulsars would be hard to identify in other galaxies. So, keeping it just to our own galaxy, it still seems unlikely that a Berserker culture could cover the distances to kill all other advanced cultures. Even a half a dozen killer cultures evenly dispersed in our galaxy couldn’t do it.
Of course then it all depends on how advanced the advanced culture is. There are, purely theoretical relying heavily on unobtanium, cheats around the light speed limit. If they’re THAT advanced then they could be wiping out civilizations. Of course they still have to find them, and that would still in many ways be hampered by the speed limit (don’t care how fast your ship goes, our radio waves are still “only” going speed of light), so they have to get in the zone.
Which is why I think distance is the true great filter. Every way we have of announcing our presence, or detecting somebody else’s, take a long time to get anyplace and would carry extremely post dated information. The way I think about it is to “assume” there’s another civilization out there 200 light years away growing their technology at an identical rate to us. Their very first experiments in wireless telegraphy haven’t gotten here yet. We’re using very dim flashlights for a very large room.