Check a mesh and locate its defects
Open an STL and see where it is broken. This finds open boundary edges, inconsistent winding, non-manifold edges and degenerate triangles, and shows each one on the model. It reads the file in your browser and uploads nothing.
Find and locate mesh defects. Your file stays in your browser.
What it looks for
Four things, and it counts each of them: boundary edges where the surface stops instead of closing, triangles wound against their neighbours, edges shared by more than two triangles, and triangles with no area. It also reports connected components and unused vertices.
Select any finding and it is drawn on the model, so you can see whether a hole is one real gap or a hundred slivers along a seam. A count on its own rarely tells you which.
What it will not tell you
Whether the part will print. Watertightness means the triangle edges form a closed, consistently oriented surface, and that is all it means. Wall thickness, clearances, overhangs and support are a slicer's job, and a model can pass every check here and still fail on the bed.
It also does not fix anything. Diagnosis and repair are separate on purpose, because most of what people call mesh repair is guesswork about what the model was supposed to be. When a defect is unambiguous, the repair tool will take it.
After you know
A hole usually belongs to the export, not the mesh, so the cheapest fix is often to re-export from the source CAD. If the original is a STEP file, STEP to STL gives you a fresh mesh with no accumulated damage.