If you have only few linkages established, you can check the i't'ems of levers _and_ of possibly linked buildings - if there's a mechanism included in a building which doesn't normally contain one (or extra mechanisms in case of levers etc.), this means the building is linked somewhere. Very early in a fort's life, when only few buildings exist/are linked, that can already give some good hints.
A lever cannot be linked again to an item it's already linked to. If you have only three bridges and _know_ each of three levers is linked to one of the bridges, you can 'q'uery the lever, order a new job - link to a bridge - and look through the linkage options on each lever. The already-linked bridge will not be offered.
There's only a very limited number of cases where this approach really works - the number of possible linkages must be quite limited for you to spot the correct 'missing' item.
A saner approach is thinking ahead: label your levers via the 'N'otes system as soon as you linked them. Hit 'N' to enter the notes menu, hover the cursor over the lever, 'p' to mark the place and 't' to enter some descriptive text. This obviously is only a 'better think of that next time' tip.
If you have DFHack installed, you can use the 'gui/mechanisms' application, which shows you where a lever is linked.
And if you have levers with inscrutable links and don't feel like digging through the link list, you can simply build a new lever and link it up properly (and put up some notes this time). For added safety, yank out the botched old levers as well. Linking up or removing linked levers doesn't activate buildings, 're-wiring' parts of your machinery is always a safe alternative to randomly pulling levers and hoping they don't do something irreversibly FUN.