I have an image with a PNG extension.
I suspect it's not really a PNG though (I think it might be a GIF).
How can I confirm an image's encoding?
NB: A solution for Windows would be preferable
You need to do a quick check of the file's internals.
The first 8 bytes of a PNG image are always
137 80 78 71 13 10 26 10
It is vanishingly unlikely that a non-malicious non-PNG has the same 8 bytes.