New to WP – Sidebar question

I am working on a site that will be working as a portfolio site, and my question has to do with the sidebar. I will be listing each of my projects under different categories such as web, print, etc. Now, when someone clicks on one of the posts for a project I would like a list of projects that are under the same category to be in the sidebar. The only problem is that more projets than not fall under more than one category.

So my question is….Is there a way to trace what path someone gets to a post so that in the sidebar the correct category will show up relating to what they wanted to look at. If so, how would you go about doing it?

Thanks