Quote Originally Posted by Deus Ex
in case it wasn't clear from the other posts - to actually SEE that your trading is working, use the "Income" icon (lower left corner) and then click on each of your provinces - if you are trading - a scroll will come up on the left side showing all the other provinces the current (one you clicked on) is trading with.

hope that helps

DE
Or right-click on your trading post - it will tell you how much it is earning that turn. (If you use DE's method you get the details, too - how much the province is trading with each other port it is connected to.)