How to find out what custom Line Styles are stored in a DGNLIB file.
Custom Line Styles can be stored in either a resource file ( *.rsc) or a in a library file (*.dgnlib ), the Line Style Editor can be used to see the custom Line Styles with *.rsc files but not for those stored in a *.dgnlib.
Open the Line Style dialog ( Element > Line Styles > Custom )
The Line Style names without brackets around them are those stored within the dgnlib file that is open, those with brackets are from other sources. Selecting the Line Style name will also show the where it is being read from above the Line Style preview.
Open the Explorer dialog and expand the File section and expand Line Styles
The Line Style names within the opened *.dgnlib will be listed.