How can I see where a mesh is open?

Question

I have a mesh that is supposedly open, but I can't see any holes in it. Is there a way I could find where they are?

Answer 1

If there's a hole in your mesh, that's someplace where the mesh is "non-manifold", and you can use a Select Non Manifold operation in Edit Mode via menu Select > Select All by Trait > Non Manifold, in either vertex or edge select mode, to quickly select all such boundaries:

You can see on the operator panel that I've disabled all but Boundaries, which gives me the borders between holes and mesh. Other aspects of being non-manifold are just as important (more important, really) than boundaries, so odds are that if you need to fix boundaries, you need to fix everything else as well-- you might consider selecting all non-manifold, regardless of subtype. (But "extend" isn't a subtype, it just means, add whatever gets selected to any existing selection.)

Once selected, you can quickly Zoom To with NumPad . (numpad period) the selected, and/or hide all but the selected (Shift+H).

Answer 2

Under the menu Edit > Preferences > Addons tab, search the addon 3D Print Toolbox and enable it.

Back in the viewport, ↹ Tab into edit mode. In the Sidebar > 3DPrint tab, Analyze panel, hit the Solid button.

It should show you "non-manifold edges" in the "Results", you can click it to select the edges:

Alternatively, if you don't want to bother fixing holes manually, you have the "Make Manifold" button under the Clean-Up sub-panel: