Open the file on 0xr.space, press More in the dock, then Info. The WATERTIGHT row says Yes when the surface is closed, Yes · faces flipped when it is closed but some faces point the wrong way, and No with the number of open edges and of edges shared by 3+ faces when it is not. VOLUME gives the enclosed volume in cm³ or litres. The check runs on the device, in a browser or on a Quest.
Corners are welded first, so an STL that repeats each vertex per face still reads as one surface. When the mesh is open or has flipped faces, the volume is written with a ~ as an estimate, and an open sheet that encloses nothing reads None. A rigged model, a splat or a point cloud reads Not a mesh. The check only starts when Info is opened.
Edges used by three or more faces, called non-manifold. A slicer cannot tell inside from outside along them.
No. Fix, then Normals rebuilds the shading from the faces as they are; it does not reverse them.
A skinned model's rest pose is not the shape on show, so it is not checked.
No. It is read and checked on the device, and nothing is sent to a server unless you create a share link yourself.