If I place a 1x1 bridge, and hook it up to a lever, how can I tell if that bridge is raised? If the lever is ON, will it always be raised (I dont think so, because I can hook it up while the lever is ON, thus flipping the bits, right?)
This is actually a problem for all bridges of length 1. The lever state cannot be relied upon to indicate the bridge state, especially if two or more levers (or other mechanisms) control the bridge. Here is the solution I have found.
There are two cases:
In one case the bridge is submerged in liquid. If this is the case then the liquid will show instead of the bridge whenever the bridge is open (that is it can be passed because it is lowered). Conversely the bridge will show instead of the fluid when it is raised.
In the other case where the bridge is on dry land you can determine the state of the bridge using meeting zones. Place a 1x1 zone on one of the tiles that the bridge occupies. If the bridge is lowered and dwarves can stand there then the zone will say that it has a tile usable as a meeting area. If the bridge is raised and behaving like a wall then the zone will have no tiles available as a meeting area. There are several other zone types that behave in the same way such as garbage dumps, pen/pasture, hospital and animal training.