A Road Tile can be in one of three states: face-down with no Shield Marker (totally unknown); face-down with a Shield Marker (known to one player); or face-up (known to both players).
This post concerns the middle case, when only one of the two players knows what's one a Road Tile (either through raiding or spying).
That player can raid the Villages on that Bay Board and know which Treasure Tile is reached by each road (white/green/blue/red). When he does so, the other player has trust that the correct Treasure Tiles are taken.
When the other player later decides to raid through such a Road Tile, he doesn't want to select a road that has already been used - there will be no Treasure Tile remaining at its end.
He could ask the first player which roads have been used, and it seems reasonable to tell. After all, the rules say, "it does not make any sense to raid an already raided Village again." There is no discussion of what happens if you try to do so through ignorance or failure to remember.
To address these issues, I propose here a simplifying player aid, that I call Taken-Road Markers. The resulting variant is consistent with the original game if there is supposed to be no memory element.
You will need at most 6 of these, 2 for each Bay Board in play. That's a nice number, because the game comes with 30 Treasure Tiles and only 24 are used in each game. The 6 unused Treasure Tiles can be used (face-down) as Taken-Road Markers.
When a player raids a Village through a Road Tile that will remain face-down after the raid, place a Taken-Road Marker on the road that was used. That way, the opponent will know which roads have been used - and which ones not to try.
After a third Village on a Bay Board has been raided (the Bay Board already had 2 Taken-Road Markers), turn the board's Road Tile face-up. Even if it would normally remain face-down (all 3 Villages were raided by the same player), there really is no more hidden information - the last road must connect to the last Village.
Whenever a Road Tile is turned face-up (including the case of the previous paragraph), check that the Taken-Road Markers connect to the Villages whose Treasure Tiles have been claimed. (If they don't, someone made a mistake.)
If a Road Tile is face-down, at most one of the players have raided through it. Thus, it is sufficient to check that the Taken-Road Markers collectively match the taken Treasure Tiles. There is no need to match them one-to-one, as the same player took all those tiles.