Hi Strube, set up a keyboard shortcut with the script shown here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

That will then give you a way to see "naked edges" in your model - those are edges that belong to only one face instead of being joined between 2 faces. Those are the edges that will prevent a model from being a fully closed and watertight solid.

- Michael

Works great. Thank you.

Now that I know they are called "naked edges", the snippet is also on Petr's site. It is called "SelectNakedEdges".