To do this you look at the last few letters after the last '.', e.g. word documents will end with '.doc' (you may not see these extensions for file types that your installation of Windows recognises - to see them: double click on 'My Computer'; select 'Options' from the 'Tools' menu; select the 'View' tab and deselect 'Hide extensions for known file types'.)
So you know what extension the file has, but still don't know how to open it. There are a number of web sites that provide lists of know file extensions. All offer a search tool, so you can just type in a file's extension and find out what programs you need to open it. So here are a few such sites:
- Wotsit.org (my personal favourite)
- FILExt
- file-extensions.org
