How can I get the depth of a jpg file?

Jpeg files don't have bit depth in the same manner as GIF or PNG files. The transform used to create the Jpeg data renders a continuous color spectrum on decompression.

I don't see the depth attribute documented anywhere in the Python Imaging Library handbook. However, it looks like only a limited number of modes are supported. You could use something like this:

mode_to_bpp = {'1':1, 'L':8, 'P':8, 'RGB':24, 'RGBA':32, 'CMYK':32, 'YCbCr':24, 'I':32, 'F':32}

data = Image.open('file.jpg')
bpp = mode_to_bpp[data.mode]