Downloading the sample is the only way to really tell, I think - I've seen books that don't show a file size actually turn out to be AZW instead of the expected Topaz. I've not seen books be a different format than their sample. Topaz samples seem to be DRM'd, too, but AZW/mobi samples are not.
There are 2 different extensions I've seen - .azw1 is what you get when you download to your Kindle, and .tpz if you download to your computer.
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