Do not want to sound weird or anything, but only good way to know is to see or know that there are dead bodies there (from the sniper).
We use to know those areas, because few things like obviously good places like intersections, open areas ,and of course common sense where “enemy” could be.
In today modern SHTF scenarios, i am not really optimistic, I mean if we are talking about professionals, and I have seen some of them, they can be really really good.
Cool game, but in real life it is a bit different, while you are trying to “spot” the sniper, you are already dead if you are doing that openly.