how to retrieve alpha channel data in format plugin

I want to ask how to get the alpha channel 's data that describle the document's transparent info, in a format plugin.

for example , when open a PNG file in Photoshop, the photoshop set FormatRecord:: planes to 3 (not 4),

in this case i can only request the RGB data.

then how can i get if the pixel is transparent in document ?  (that is 0 - completely transparent, 255 - completely opaque)

Thanks~