Suppose you are working with Italian and that your tags include stuff like
<VER:ppre>
<VER:pper>
<NOM>
<ADJ>
(where the first two are specific verb forms) and you want to find all the <VER:ppre> forms.
Put the search word like this:
(Why the * at left and right? Because the tag may be right next to a word on one side or the other!)
and after you have selected the language and the texts, you should be able to get something like this:
Page url: http://www.lexically.net/wordsmith/Handling_BNC/index.html?concordancing_just_the_tags.htm