When shooting for a culture victory, I keep reading that you need to set up international trade routes to help improve tourism. However only some civs seem to show the +25% tourism bonus after I setup a trade route with them. Even when looking at available routes it says that I will have a +25% tourism bonus on some, but then after the route is created it doesn't actually show that anymore. I have trade routes up with 4 different civs, but only one of them is showing the +25% bonus on my routes list.
The better place to find this information is under the victory screen. There's a tab for the culture victory, where you can hover over the incoming tourism icon from each civ (briefcase in the second column) and see the +/- that is affecting your tourism with them.
EDIT: With Screenshots of the suggestion! Here's what I'm talking about, and this may be a bug. The trade screen doesn't always correctly state when the trade route is providing tourism.