Speaking about Joomla specifically, it has a folder named templates in which templates are kept. I usually find out about the templates using
However, if the designer changes the template directory name or removes the meta data from within the css files or builds his/her own css from ground up. There wont be much left to identify the template. Hence you will have to do it the hard way , by identifying the CMS used and then browsing thru CMS specific templates. :)
Hope this helps you!