Unfortunately I find that this is more difficult to deal with than it should be. No one wants to put in writing that they were asked for a "bribe" as often people are intimidated by the super or fear reprisals.

My feeling is encourage people to put it in writing. If it is happening often enough perhaps you can build a paper trail showing it is a repeated action.

To me there is a very big difference between a super asking for money before a job, and a tip being given at the end of a job.

One other thing that you might watch for in a building is reports that the super "pushes" certain contractors on the shareholders when they mention renovations.