Without ships of their own the only trade they should get is local trade,ie within their own faction, like one of your interior provinces that has no sea access can only trade within the province, not globally. It would only be a minimal amount. There must be an unbroken/unblockaded string of ships from port to port for trade between factions.
At least that is how it's supposed to work.
How can you tell your opponent is receiving trade with other factions? As far as I know there is no way to see your opponents trade parchment. One of the AI problems is that they sometimes don't set up viable trade routes with other factions, they leave breaks in the chain of ships required, ie unoccupied sea areas, to complete a trade route.
You do know you can check your trade to every port by first clicking on the "stack of coins" button, this brings up a parchment on the right listing all your provinces with their income + support costs for your navy.
Then clicking on the name of the province on that list, Flanders for example,brings up another parchment on the left showing every port you export trade goods to, if the AI has a viable trade route to Flanders it will also show on this parchment as an import, took me awhile to discover that one myself, by accident in fact lol.
As far as I know there are no overland trade routes between factions, and the only trade within your own faction only takes place in the province that produces the trade good. Something I hope they fix in the patch. You should be able to trade a commodity to every province you control and can reach overland without entering another factions territory.

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Diplomacy is the art of telling someone to go to hell so that they look forward to making the trip.

[This message has been edited by Hosakawa Tito (edited 10-29-2002).]