How to tell if a .gz file is really gzipped?

Question

What a nice command line trick to tell if a .gz file is really gzipped or just named that way?

Answer 1

You can use the file command to do this. It will look at the magic numbers in the file and tell you what type it really is.

Answer 2

If it is gzip'd the first two bytes are 0x1F and 0x8B.

To get more complicated than that you will need to understand the file format. See here: http://www.gzip.org/zlib/rfc-gzip.html and go down to "Member header and trailer" section where you can identify a gzip by its 10 byte header.