To directly answer the question, road maintenance is listed as "Tile Improvement Maintenance", when you mouse over your treasury/income in the top left. You can see a circled example in the screenshot below. As for the details:
Roads cost to maintain both inside and outside your borders
This is contrary to what bwarner writes, so ... Pics Or It Didn't Happen!
Exibit 1: Prince Difficulty, some roads in, some out.
As you can see, I have 4 roads in my territory, and one outside, costing me a total of 5 maintenance. I have no other roads anywhere else. I wish I had taken a screenshot of the previous turn. The next time I find myself in this situation, I'll take a pair of screenshots and replace this single picture. Either way, remember you can always try it yourself to be sure. YaY Science!
Exibit 2: Settler Difficulty, all roads out.
Thanks to @DMA57361's questions, I checked a different difficulty and a different number of roads outside the borders, namely all of them. On Settler difficulty, building roads only outside of my borders, I still ended up with maintenance for the roads, so clearly they do cost.
However, either the difficulty or being outside changed the cost/tile, meaning I needed 3 tiles to produce 1g maintenance. This requires more testing, and makes the screenshots I posted for the next section suspect. (I made them from the same save game)
Roads DO NOT cost to maintain inside another nation's borders
Edit: This section is suspect until I complete further testing, since one road did not cause me maintenance. My mistake for not setting up a proper control. This still may be completely true, but I need to test on different difficulties and compare with similar numbers of roads in my borders and in neutral territory.
In the game these two pictures originated from, I had no roads, and built a road inside each another civilization's borders, and a city-state's borders. I can't verify that they were costing any gold to the land's owner (Rome's income was fluctuating quite a bit), but I can say for sure they were costing me nothing. This is likely what bwarner saw that caused the error.
There may be other effects that change which roads cost to maintain, but these seem to be the major ones. Anyone found anything not explained by these.
PS As with any screenshot heavy post, I do my best to crop/scale the images to reasonable sizes without losing the important parts, such as the circled text. They still end up crowding the page, for which I'm sorry.