TImageList quetsion

[Kikimorcho]

Hello, I want to ask if there is a way to determine whether an image in the imagelist is a bitmap or png for example. Or to be more precise, whether the image has an alpha channel or not.
When set ColorDepth to cd32Bit, bitmaps loaded have alpha 0, and pngs - well, whatever alpha they have. I get a image from the imagelist and draw it according the alpha, works great for pngs, but bitmaps become fully transparent.
I can check the alpha of all pixels, and if 0 - treat image as bitmap, but was wondering if there is a better way?