Often it is helpful to know who is trading with whom. If you know that, then you know which AI to bribe to start wars with to eliminate trade routes. The goal is to get 100% of an aliens trade routes to you.
My problem is when I have isolated a major AI but when I click trade bar, I find out 24% of the AI income is from trade but only 12% is from me. I can see what sort of trade of going on with the majors but I can't see what trade is going on with the minors, can I? Is there a way to know what sort of trade routes the majors have to minors?
Advanced espionage will allow you to see their trade routes, both coming and going. That's only helpful later in the game, though.
You can also tell who has established trade and solid relations by who they WON'T agree to attack for you. Then it's too late, though. So again, not much help for you.
Thanks for the input. Now, I just try to get the majors to war with all the minors too. I pick the minor closest to the major first, which is sort of logical. And, sometimes I don't put my cash cow (Alexians and rarely Carinoids) to war so I can keep those charge card payments rolling in.