Getting units from ISpatialReference of ArcObjects?

Linear units can be obtained from the spatial reference only if it is a projected coordinate system. So, you need to cast the spatial reference to IProjectedCoordinateSystem and access its IProjectedCoordinateSystem.CoordinateUnit property.

But if the spatial reference is a geographic coordinate system, its units are angular and accessed similarly through IGeographicCoordinateSystem.CoordinateUnit.