I checked this website and looked up a joomla website but it didnt give any details for the cms. I checked a wp blog and it showed Content Management System: wordpress
Most of the times, you can see a joomla website by:
-The sef urls might not be enabled so you will see something like com_content when you click to read a post (a decent portion of userrs do not enable sef urls!!)
-In the page source, you will definitely see either /component or for sure /module which is the same as plugin for wordpress (pretty much). This is something that you would definitely see if you browse a couple of pages
-If they have eshop, 90% that they will use virtuemart so site:xx.xxxx virtuemart should give you results
-Finally if they use commercial templates and you know some well known (ja,jv etc etc) you will see it in the page source. (/templates/ja_xxx ) etc