You are asking for inside information which is between the broker and the agent.
If your the seller that information should be on your contract , if your the buyer there's no real way for you to find out that info is not public to buyers, but those percentages are not set in stone and can be changed or adjusted
Some states - Arizona is one - now have the co-broke listed on the listing agreement itself.
Follow-up question would be why you were looking for this information? If it's with the idea that you can "save" that co-broke by going through the listing agent, that's most often not the case as the total commission being charged by the listing brokerage isn't generally contingent upon a second agent and generally isn't reduced.
I say generally because I do reduce my commission when there's not another agent, but that's not at all a uniform practice.
Before I answer your question, I'm curious to know what you plan to do with this information.
It is important, but the pathway to revealing this is clearly defined, and apparently not available to you. So, what's up?
Are you selling via a flat fee broker and want to be competive?
Are you thinking about DIY?
Are you second guessing your agent?
Are you gathering informaton for an NYT article?