I'm tracing from the Position which should be checked to the middle of the object.

If it hits nothing it is inside.
And as long as all normals pointing away from the center and the center is inside the mesh, too this works.

For more complex models I'd go for your solution.

But as I already said: c_trace isn't the right way to go if I want to do this once per frame.

It would be nice If it works as I want, but It doesn't matter.
It's just a kind of test-project I wanted to do, but obviously it wont work fast enough.