Do you mean the depth of the cover? If so get hold of a catalogue from the likes of Norinco or PAM. They will give you all the dimensions of drain covers.
Or do you mean cover levels? If so, you can get a level and go and check them yourself. If that is not practical contact the local water company. Typically they will have a GIS system which 'should' contain record data for manholes, such as cover levels. If not they might be able to provide you with copies of the manhole record cards (they may be called something else). A word of warning though, the record survey data cannot be relied upon without a check of some sort.
