Checking for closed mesh

Question

I'm using Blender to create meshes I want to fracture in UE4. For this they need to be closed, otherwise their inner is not fractured correctly.

I'm creating a basic house from a cube. First I extruded the "door" inwards and made the vertices there so that it's closed. I exported it and it can be fractured correctly.

Now I tried to extrude two windows, but when I fracture them, they are fractured like it's not closed.

Anyone knows why or how I can check if it's closed?

The windows I extruded:

Other view:

Wireframe:

Answer 1

I created a facsimile of your scene to illustrate how this can be done. Here is what I start with.

Next, I create some non-manifold geometry so that I can later illustrate how to expose it. In Edit Mode, I select the edges highlighted below, and peform an Edge Split.

Next, while still in Edit Mode, I go to the Select menu, and Choose Select All by Trait > Non Manifold. This will select all geometry that is not closed.

Very handy. Here is the result.