There needs to be some sort of notification that you have trade routes available. I constantly forget to check when the seasons change.
I think once you train the association that once the seasons change you should check for new trade routes this is not much of an issue, but having a "New trade routes are available" prompt wouldn't hurt.
Having a number on the trade route icon on the top-right with available routes: 0, 1, 2, 3 depending on your cap would work well here as well. Then you can know at a glance the state of the routes.
Currently I use the route completion notification as my reminder, but that's quite inefficient and it made me completely ignore the auto-complete feature. The latter which, by the way, has anti-synergy with incoming order that may require you to complete routes.