How to identify a NSData's image format?

If I get a NSData which I know it's a image's data.But I don't know what format it is. So how can I identify which image format is it?Jpeg or PNG?

PS:iOS

Solution

You could look at the first bytes and make a guess. There are many lists of magic numbers available on the internet, e.g. http://www.astro.keele.ac.uk/oldusers/rno/Computing/File_magic.html.