If you use Sketchup seriously for any amount of time then you are likely familiar with how ridiculous it gets when Sketchup tells you that your model is not a "solid group" and therefore not watertight (and therefore not ready to be 3d printed). The traditional way of working through this has been to export the mesh as a .dae file, open it up in another program called Mesh Lab, export from there as an .stl, and then open up the .stl in a program called Accutrans which can then find where the fault is in your mesh.. So something akin to insanity, and would cost you $20 to buy Accutrans.
So let's stop the insanity.
Thanks to thomthom, a global moderator and plugin writer over at sketchucation.com, there is now a plugin called Solid Inspector that is FREE and it finds the problems in your geometry quickly and easily by highlighting and circling the problems in your mesh, and it all works from right within Sketchup. It is a thing of beauty.
To install this you need to do three things.
1. Sign up on sketchucation.com (you can't download the plugins until you do).
2. Install the TT_Lib2 plugin found at the address listed below. This is a background utility plugin that is needed by some Sketchup plugins in order for them to work.
3. Install the Solid Inspector plugin found at the address listed below.
To use it, group your geometry, go to your tools drop down menu and choose Solid Inspector. Wallah, problems are highlighted and circled.