OK I'm using now EvilSOB's method.
I created a model and checked a cube with 20*20*20 single traces... which works fine
but sometimes there are points outside of the mesh detected as inside of it... (pic)

Another thing is, that this c_trace-method only works it I add wait(...); functions between every trace.
Without wait()s the detection goes completely wrong.

And it would be nice if this could be checked once per frame.
But even if it would work without the wait(); functions it would be too slow. (Manual>c_trace>Speed: Slow)

Looks like this won't work with 3DGS.
Anyway, thaks a lot for your nice replys!

Kartoffel