Local? country? continental? global? What is the coverage of the map? Did you ask a cartographer or map librarian for their opinion?
If it is in a projection common to your area, start with the ones commonly used. Remove newer projections and those with specific datums which predate your map to narrow down the list.
From there.... guess which is the closest.
If you intend to just georeference it, it really doesn't matter (much) since you will be providing the map to real world coordinate control points during the registration process. How well it goes, depends on the control point placement. Don't expect a decent result if the coordinate system of the map isn't remotely close to the one you are trying to georeference to.