Dont they have their own simulator mode or similar, like the garmin GPS, or may be you could use a program like oziexplorer to export nema sentances in demo mode.
Check out: http://www.nmeatool.de/ (it also has en english language version of the website.
Had a brief look at it, but never used it as it seemed simpler to do what I needed with a bit of programming. SO I can't say how well it works.