As far as I know there are no lists avalible which code is linked to which polyline/polygone etc so some trial and error may be neccesary
Yes, no such list exists ( and cannot exist because these associations between objects and codes are not always the same, they often depend on the mapset...)

City Navigator are now NT maps , so there is, AFAIK, no simple way to know which code is linked to a polygone/polyline...

Personally I use special typ files to do that: I create typ files including a lot of polylines ( for example all polylines from 0x10000 to 0x10F1F), so that every polyline has an unique color, whose RGB code "includes" the type and the subtype of the line (for example , color #B10713 stands for type=0x107 and subtype=0x13).
Then you open Mapsource(or Basecamp) , don't forget to disable shading, and you look at the color of the line you want to discover the code. The color gives you the type and subtype ( because in my typfiles, the color code includes the the type and subtype of the polyline).

To do this, you can use TYPViewer (of course! ;-) ) : go to "Tools/Special typs for types identification" and create a typ file (or several typ files if you have no idea of the range where the type of the polyline stands).
Then associate this typ file with your mapset (use again TYPViewer : "Tools/Install a typ file" ) and then use the color picker of TYPViewer ("Tools/color identifier" and don't forget to clear Mapsource/Basecamp cache - TYPViewer can do it to)) : you will see the type and the subtype of your polyline. (if not, the typ file you have create may not cover the right range of types; try another)