Determining if file is valid image file

[André]

Other than installing PIL, is there a "simple" way using Python only
to determine if a file is a valid image file?

I'd be happy if I could at least identify valid images files for gif,
jpeg and png.   Pointers to existing modules or examples would be
appreciated.

The reason why I'd prefer not using PIL is that I'd like to bundle
such a function/module in my app.

André