If you have the actual game hooked up to your HDTV, then you can tell if it's 480i or 480p by checking various things. On some HDTVs, the TV can show you the resolution which is being fed to the TV. On other HDTVs, one has to figure this out by less direct methods. For example, on my Sony GWIII, there are a set of deinterlacing setings (for adjusting "Reality" vs. "Clarity" and fiddling with CineMotion); these controls are inactive when displaying 480p content.
I don't think anyone from Nintendo ever stated that the Wii would de-interlace games to 480p. What was stated was that Wii games would all be written to make use of progressive scan, i.e. they'd all support it. We seem to have evidence that this is not true at all, that Nintendo is not enforcing this and is probably only guaranteeing this for their first party titles (similar to the Gamecube situation).
If you have a 4x3 TV and you're playing a Wii game which supports 16x9, the game simply uses a normal 4x3 field of vision. There's no letterboxing; remember that the Wii is designed to be used with a 4x3 standard television, so 16x9 is actually the non-standard use case, not the other way around as with the Xbox 360.