I just did a quick search, and unfortunately I can not seem to find where AutoCAD is storing that info. If you run the Linetype dialog in AutoCAD and click 'Load' it obviously remembers the last file that was used, but I have searched all over and can't find where. It does not seem to be in the registry or the system variables. I also do not see anything in either the COM or managed API's.
The only thing I might suggest is that you look in the LinetypeTable and see if there are ISO or ANSI linetypes currently loaded. (The ISO linetypes I have seen before all say ACAD_ISO in the name). Of course, you would be making an assumption, and could make an incorrect assumption, so maybe your dialog is the best way to go.
Dave O.