debugging – OpenSCAD Troubleshooting Method : How (strategies or tools) can I find and plug a “leak” in a 3D model or in an included STL file?

I receive occasional “ERROR: The given mesh is not closed! Unable to convert to CGAL_Nef_Polyhedron.” messages from openSCAD. I have a hard time finding the origin of the problem but I suspect it might come from STL files I included in my model⁽¹⁾.

So, outside the recommended best practice in the code writing, to avoid shared surfaces, what are the strategies or tools I can use to find WHERE those leaks are (and how can I “plug” them) ?

(1) I made those STL file myself with openSCAD, from other STL files I transformed with tinkercad, and the making included taking a cut to extract writings (both sides : writing + negative of the writing), combine them with cones (minkowski), etc. – and my code itself is quite complex. So there are many possible sources for this problem and I’m looking for ways to isolate them.

Edit : Someone on a group suggested the Meshlab software to analyse the STL files.