Some file formats have a hint at the beginning (PDF Files start with a string like '%PDF-1.3') but many file formats don't really have a good visual indication of what the content of the file is. If you are lucky, your file will have a magic number indicating what type of file it is. You can trudge through that and see if you can find your answer.http://en.wikipedia.org/wiki/Mag...If you are lucky enough to have a text file on your hands, you can open it with your trusty notepad like application and examine the contents. Hopefully you will get some hint as to what the file is supposed to be. Many text based Unix-like files have a shebang at the start of the file indicating what program is used to interpret the file. This looks something like '#!/bin/sh'. (This is a magic number too!)If you are faced with an unknown binary blob that doesn't give any clue to the nature of its contents, unfortunately, trial and error is your only route.If your operating system is simply hiding the extension then User-13375181939089766119's answer is sufficient.