I think of it this way....
There are two types of foul. Totally obvious fouls (i.e. no rail, complete misses, etc.) and close call fouls.
If I'm playing someone and they are miss me making a totally obvious foul because they just aren't paying attention, they too bad for them. (Unless there are a very new player) This is in pool and I'm doing what 99% of player would do. Close fouls I'll call them as I see them... agree on the correct call/way I see them. BUT, it is always on the opponent to call over someone to watch a close shot if they think there is a chance of a foul.
I also play snooker and the ethics are a little different there, so I'll call everything... also a foul in snooker usually isn't as damaging to you, so calling them on yourself is no big deal.
Basically the it is completely different between pool and snooker. Pool, it's up to your opponent to call them. Snooker it is up to you.