How do you know the ligand contributions to the oxidation states of coordination complexes?

I am refreshing some of my inorganic chemistry because I noticed I got a question on the oxidation state of a coordination complex wrong. I understand how to find the oxidation states now (e.g. OXm = complex charge - ligand contributions), but the only thing I am not sure about is how to know the ligand contributions. For neutral ligands such as H2O, NH3, or CO it is zero. And for anion (-type) ligands, such as CO- and Cl-, it is -1. But what about other ligands, such as pi-bonding ligands like benzene? I haven't found an example of those, but since benzene is neutral is it just zero? Do the ligand contributions mainly go by charge (neutral or ionic) or is there a table for the ligands (such as with the 18e- rule)? Thanks!