You should be able to ask the current owner for their past expenses (if not bank owned).
It is going to depend on where the property is located, is there yards to maintain, are utilities separate or does landlord pay, is there an HOA, how is sewer billed, garbage service. Then of course there is insurance, property taxes, maintenance, property management expenses, mortgage if you are purchasing outright. Income taxes.
You should be working with an experienced agent that knows investment properties. There are also a lot of books on investment properties and property management. Visit the library or your local association of Realtors book store. There is good info on landlord/tenant rights on the DRE website, http://www.dre.ca.gov