I guess you are looking for a Windows related answer, but on Linux, to detect the prescence of the c based gtk+ libs, which Wx would probably be using, you would just check the contents of the pkgconfig directories. The pkgconfig directories maintain the versions and locations of installed gtk+ libs and headers, which are installed in each library location. So you may need to check all paths in your ENV variable LIBPATH.
in reply to how to detect gtk installed or not
Run the command pkg-config (notice the dash) for a commandline tool to search it.
I have no idea where MS Windows puts those files.