This is really an ATC question. I just file a plan, talk to centre and, generally, they provide information on traffic, etc.
Many a time OCTA I've become aware of (potentially) conflicting traffic and made up my mind early to be the master of my own fate (either asked the other aircraft to climb or descend or climbed/descended myself).
I think its less a question of separation standard and more about what distance/altitude/time between aircraft requires ATC to mention other traffic to you.
Certainly happy to discuss. Good question...fertile ground for discussion.
HP