In my engine I have a need to be able to detect DXT1 textures that have texels with 0 alpha (e.g. a cutout for a window frame). This is easy for textures I compress myself, but I'm not sure about textures that are already compressed.
Is there an easy way to tell from the header whether a DDS image contains alpha?
As far as I know, there's no way to tell from the header. There's a DDPF_ALPHAPIXELS flag, but I don't think that will get set based on what's in the pixel data. You'd need to parse the DXT1 blocks, and look for colours that have 0 alpha in them (making sure to check that the colour is actually used in the block, too, I suppose).
I agree with the accepted answer. Your job may be made a bit easier by using the "squish" library to decompress the blocks for you.
http://www.sjbrown.co.uk/?code=squish