A bit more information would help - what format are your maps? Are they hard copy (paper), PDF, or map documents (.mxd file)? Do you have access to the digital elevation model from which the contours were created?
If you have read access to the source DEM used to create the contours I'd examine the coordinate systems tab in the dataset properties in ArcCatalog. I think that will tell you what vertical datum was used.
Hope this helps.